Editing Diamond (section)

== Properties ==
{{Main|Material properties of diamond}}
Diamond is a solid form of pure carbon with its atoms arranged in a crystal. Solid carbon comes in different forms known as [[allotrope]]s depending on the type of chemical bond. The two most common [[allotropes of carbon|allotropes of pure carbon]] are diamond and [[graphite]]. In graphite, the bonds are sp<sup>2</sup> [[orbital hybridisation|orbital hybrids]] and the atoms form in planes, with each bound to three nearest neighbors, 120 degrees apart. In diamond, they are sp<sup>3</sup> and the atoms form tetrahedra, with each bound to four nearest neighbors.<ref>{{cite book | vauthors = Delhaes P |chapter=Polymorphism of carbon | veditors = Delhaes P |title=Graphite and precursors |date=2000 |publisher=Gordon & Breach |isbn=978-90-5699-228-6 |pages=1–24}}</ref><ref>{{cite book | vauthors = Pierson HO |title=Handbook of carbon, graphite, diamond, and fullerenes: properties, processing, and applications |date=2012 |publisher=Noyes Publications |isbn=978-0-8155-1739-9 |pages=40–41}}</ref> Tetrahedra are rigid, the bonds are strong, and, of all known substances, diamond has the greatest number of atoms per unit volume, which is why it is both the hardest and the least [[compressibility|compressible]].<ref>{{cite book | vauthors = Angus JC |chapter=Structure and thermochemistry of diamond |pages=9–30 | veditors = Paoletti A, Tucciarone A |title=The physics of diamond |date=1997 |publisher=IOS Press |isbn=978-1-61499-220-2}}</ref><ref name=ChemThermo>{{cite book | vauthors = Rock PA |title=Chemical Thermodynamics |date=1983 |publisher=University Science Books |isbn=978-1-891389-32-0 |pages=257–260}}</ref> It also has a high density, ranging from 3150 to 3530 kilograms per cubic metre (over three times the density of water) in natural diamonds and 3520&nbsp;kg/m{{sup|3}} in pure diamond.<ref name=mindat>{{cite web |publisher=Mindat |title=Diamond |url=https://www.mindat.org/min-1282.html |access-date=July 7, 2009 |archive-date=May 6, 2009 |archive-url=https://web.archive.org/web/20090506083109/http://www.mindat.org/min-1282.html |url-status=live }}</ref> In graphite, the bonds between nearest neighbors are even stronger, but the bonds between parallel adjacent planes are weak, so the planes easily slip past each other. Thus, graphite is much softer than diamond. However, the stronger bonds make graphite less flammable.<ref>{{cite journal |vauthors=Gray T |title=Gone in a Flash |url=https://www.popsci.com/diy/article/2009-08/burn-diamonds-torch-and-liquid-oxygen/ |journal=Popular Science |date=October 8, 2009 |access-date=October 31, 2018 |archive-date=March 7, 2020 |archive-url=https://web.archive.org/web/20200307065622/https://www.popsci.com/diy/article/2009-08/burn-diamonds-torch-and-liquid-oxygen/ |url-status=live }}</ref>

Diamonds have been adopted for many uses because of the material's exceptional physical characteristics. It has the highest [[thermal conductivity]] and the highest sound velocity. It has low adhesion and friction, and its coefficient of [[thermal expansion]] is extremely low. Its optical transparency extends from the [[far infrared]] to the deep [[ultraviolet]] and it has high [[optical dispersion]]. It also has high electrical resistance. It is chemically inert, not reacting with most corrosive substances, and has excellent biological compatibility.<ref>{{cite book | vauthors = Chen Y, Zhang L |title=Polishing of diamond materials: mechanisms, modeling and implementation |url=https://archive.org/details/polishingdiamond00chen |url-access=limited |date=2013 |publisher=Springer Science & Business Media |isbn=978-1-84996-408-1 |pages=[https://archive.org/details/polishingdiamond00chen/page/n9 1]–2}}</ref>

=== Thermodynamics ===
[[File:Carbon-phase-diagramp.svg|thumb|Theoretically predicted [[phase diagram]] of carbon]]
The equilibrium pressure and temperature conditions for a transition between graphite and diamond are well established theoretically and experimentally. The equilibrium pressure varies linearly with temperature, between {{val|1.7|ul=GPa}} at {{val|0|u=K}} and {{val|12|u=GPa}} at {{val|5000|u=K}} (the diamond/graphite/liquid [[triple point]]).<ref name=Bundy>{{cite journal | vauthors =  Bundy P, Bassett WA, Weathers MS, Hemley RJ, Mao HK, Goncharov AF |title=The pressure-temperature phase and transformation diagram for carbon; updated through 1994 |journal=Carbon |date=1996 |volume=34 |issue=2 |pages=141–153 |doi=10.1016/0008-6223(96)00170-4|bibcode=1996Carbo..34..141B }}</ref><ref>{{cite book| vauthors = Wang CX, Yang GW |chapter=Thermodynamic and kinetic approaches of diamond and related nanomaterials formed by laser ablation in liquid| veditors = Yang G |title=Laser ablation in liquids: principles and applications in the preparation of nanomaterials |date=2012 |publisher=Pan Stanford |isbn=978-981-4241-52-6 |pages=164–165}}</ref> However, the phases have a wide region about this line where they can coexist. At [[standard temperature and pressure]], {{convert|20|C|K}} and {{convert|1|atm|MPa}}, the stable phase of carbon is graphite, but diamond is [[metastable]], with a significant kinetic energy barrier that the atoms must overcome in order to reach the lower energy state,<ref name=baird>{{cite web | title=Why do diamonds last forever? | website=Science Questions with Surprising Answers|first=Christopher S.|last=Baird|publisher=West Texas A&M University| date=17 December 2013 | url=https://www.wtamu.edu/~cbaird/sq/2013/12/17/why-do-diamonds-last-forever/}}</ref> and its rate of conversion to graphite is negligible, with a timescale of millions to billions of years.<ref name=ChemThermo/><ref name=baird/> However, at temperatures above about {{val|4500|u=K}}, diamond rapidly converts to graphite. Experiments have found that diamond in the presence of {{chem2|H2O}} passes through an intermediate linear carbon phase.<ref>{{cite journal | last1=O'Bannon | first1=E. | last2=Xia | first2=G. | last3=Shi | first3=F. | last4=Wirth | first4=R. | last5=King | first5=A. | last6=Dobrzhinetskaya | first6=L. | title=The transformation of diamond to graphite: Experiments reveal the presence of an intermediate linear carbon phase | journal=Diamond and Related Materials | volume=108 | date=2020 | doi=10.1016/j.diamond.2020.107876 | doi-access=free | page=107876 | bibcode=2020DRM...10807876O | url=https://www.osti.gov/servlets/purl/1631913}}</ref>

Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at {{val|2000|u=K}}, a pressure of {{val|35|u=GPa}} (about 350,000 standard atmospheres) is needed.<ref name=Bundy/>

Above the graphite–diamond–liquid carbon triple point, the melting point of diamond increases slowly with increasing pressure; but at pressures of hundreds of GPa, it decreases.<ref>{{cite journal | vauthors = Wang X, Scandolo S, Car R | title = Carbon phase diagram from ab initio molecular dynamics | journal = Physical Review Letters | volume = 95 | issue = 18 | pages = 185701 | date = October 2005 | pmid = 16383918 | doi = 10.1103/PhysRevLett.95.185701 | bibcode = 2005PhRvL..95r5701W }}</ref> At high pressures, [[silicon]] and [[germanium]] have a BC8 [[Cubic crystal system|body-centered cubic]] crystal structure, and a similar structure is predicted for carbon at high pressures. At {{val|0|u=K}}, the transition is predicted to occur at {{val|1100|u=GPa}}.<ref>{{cite journal | vauthors = Correa AA, Bonev SA, Galli G | title = Carbon under extreme conditions: phase boundaries and electronic properties from first-principles theory | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 103 | issue = 5 | pages = 1204–1208 | date = January 2006 | pmid = 16432191 | pmc = 1345714 | doi = 10.1073/pnas.0510489103 | doi-access = free | bibcode = 2006PNAS..103.1204C }}</ref>

Results published in ''[[Nature Physics]]'' in 2010 suggest that, at ultra-high pressures and temperatures (about 10&nbsp;million atmospheres or 1&nbsp;TPa and 50,000&nbsp;°C), diamond melts into a metallic fluid. The extreme conditions required for this to occur are present in the [[ice giant]] [[planet]]s [[Neptune]] and [[Uranus]], both of which are made up of approximately 10 percent carbon and could hypothetically contain oceans of liquid carbon. Since large quantities of metallic fluid can affect the magnetic field, this could serve to explain why the geographic and magnetic poles of the two planets are not aligned.<ref>{{cite news |title=Diamond oceans possible on Uranus, Neptune | vauthors = Bland E |newspaper=Discovery News |date=January 15, 2010| archive-url=https://web.archive.org/web/20120311163132/http://news.discovery.com/space/diamond-oceans-jupiter-uranus.html | archive-date=March 11, 2012|url=http://news.discovery.com/space/diamond-oceans-jupiter-uranus.html |access-date=January 16, 2010}}</ref><ref>{{cite journal |title=Diamond: Molten under pressure |vauthors=Silvera I |journal=Nature Physics |volume=6 |pages=9–10 |year=2010 |issue=1 |bibcode=2010NatPh...6....9S |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:9121282 |doi=10.1038/nphys1491 |s2cid=120836330 |access-date=November 9, 2020 |archive-date=July 30, 2022 |archive-url=https://web.archive.org/web/20220730040414/https://dash.harvard.edu/handle/1/9121282 |url-status=live }}</ref>

=== Crystal structure ===
{{See also|Crystallographic defects in diamond}}
[[File:Diamond structure.gif|thumb|Diamond unit cell, showing the tetrahedral structure]]
The most common crystal structure of diamond is called [[diamond cubic]]. It is formed of [[unit cell]]s (see the figure) stacked together. Although there are 18 atoms in the figure, each corner atom is shared by eight unit cells and each atom in the center of a face is shared by two, so there are a total of eight atoms per unit cell.<ref>{{cite book | vauthors = Rajendran V |title=Materials science |date=2004 |publisher=Tata McGraw-Hill Pub |isbn=978-0-07-058369-6 |page=2.16}}</ref> The length of each side of the unit cell is denoted by ''a'' and is 3.567&nbsp;[[angstrom]]s.<ref name=Ashcroft>{{cite book | vauthors = Ashcroft NW, Mermin ND |title=Solid state physics |date=1976 |publisher=Holt, Rinehart and Winston |isbn=978-0-03-083993-1 |page=[https://archive.org/details/solidstatephysic00ashc/page/76 76] |url-access=registration |url=https://archive.org/details/solidstatephysic00ashc/page/76 }}</ref>

The nearest neighbor distance in the diamond lattice is 1.732''a''/4 where ''a'' is the lattice constant, usually given in Angstrøms as ''a'' = 3.567&nbsp;Å, which is 0.3567&nbsp;nm.

A diamond cubic lattice can be thought of as two interpenetrating [[face-centered cubic]] lattices with one displaced by {{frac|1|4}} of the diagonal along a cubic cell, or as one lattice with two atoms associated with each lattice point.<ref name=Ashcroft/> Viewed from a {{math|&lt;1 1 1&gt;}} [[Miller index|crystallographic direction]], it is formed of layers stacked in a repeating ABCABC ... pattern. Diamonds can also form an ABAB ... structure, which is known as hexagonal diamond or [[lonsdaleite]], but this is far less common and is formed under different conditions from cubic carbon.<ref>{{cite book |chapter=Molecular models of porous carbons| vauthors = Bandosz TJ, Biggs MJ, Gubbins KE, Hattori Y, Iiyama T, Kaneko T, Pikunic J, Thomson K | veditors = Radovic LR |title=Chemistry and physics of carbon |volume=28 |date=2003 |publisher=Marcel Dekker |isbn=978-0-8247-0987-7 |pages=46–47}}</ref>

=== Crystal habit ===
[[File:Diamond face trigons scale.jpg|thumb|alt=A triangular facet of a crystal having triangular etch pits with the largest having a base length of about {{Convert|0.2|mm}}|One face of an uncut octahedral diamond, showing trigons (of positive and negative relief) formed by natural chemical etching]]
Diamonds occur most often as [[euhedral]] or rounded [[octahedron|octahedra]] and [[crystal twinning|twinned]] octahedra known as ''[[macle]]s''. As diamond's crystal structure has a cubic arrangement of the atoms, they have many [[facet]]s that belong to a [[cube (geometry)|cube]], octahedron, [[rhombicosidodecahedron]], [[tetrakis hexahedron]], or [[disdyakis dodecahedron]]. The crystals can have rounded-off and unexpressive edges and can be elongated. Diamonds (especially those with rounded crystal faces) are commonly found coated in ''nyf'', an opaque gum-like skin.<ref>{{cite book | vauthors = Webster R, Read PG |title=Gems: Their sources, descriptions and identification |edition=5th |page=17 |publisher=[[Butterworth-Heinemann]] |location=Great Britain |year=2000 |isbn=978-0-7506-1674-4}}</ref>

Some diamonds contain opaque fibers. They are referred to as ''opaque'' if the fibers grow from a clear substrate or ''fibrous'' if they occupy the entire crystal. Their colors range from yellow to green or gray, sometimes with cloud-like white to gray impurities. Their most common shape is cuboidal, but they can also form octahedra, dodecahedra, macles, or combined shapes. The structure is the result of numerous impurities with sizes between 1 and 5 microns. These diamonds probably formed in kimberlite magma and sampled the volatiles.<ref name=Cartigny>{{cite journal | vauthors = Cartigny P, Palot M, Thomassot E, Harris JW |title=Diamond Formation: A Stable Isotope Perspective |journal=Annual Review of Earth and Planetary Sciences |date=May 30, 2014 |volume=42 |issue=1 |pages=699–732 |doi=10.1146/annurev-earth-042711-105259 |bibcode=2014AREPS..42..699C|doi-access=free }}</ref>

Diamonds can also form polycrystalline aggregates. There have been attempts to classify them into groups with names such as [[boart]], [[ballas]], stewartite, and framesite, but there is no widely accepted set of criteria.<ref name=Cartigny/> [[Carbonado]], a type in which the diamond grains were [[sintering|sintered]] (fused without melting by the application of heat and pressure), is black in color and tougher than single crystal diamond.<ref>{{cite journal | vauthors = Fukura S, Nakagawa T, Kagi H |title=High spatial resolution photoluminescence and Raman spectroscopic measurements of a natural polycrystalline diamond, carbonado |journal=Diamond and Related Materials |date=November 2005 |volume=14 |issue=11–12 |pages=1950–1954 |doi=10.1016/j.diamond.2005.08.046 |bibcode=2005DRM....14.1950F}}</ref> It has never been observed in a volcanic rock. There are many theories for its origin, including formation in a star, but no consensus.<ref name=Cartigny/><ref>{{cite journal | vauthors = Mohammad G, Siddiquei MM, Abu El-Asrar AM | title = Poly (ADP-ribose) polymerase mediates diabetes-induced retinal neuropathy | journal = Mediators of Inflammation | volume = 2013 | issue = 2 | pages = 510451 | year = 2006 | pmid = 24347828 | doi = 10.1086/510451 | pmc = 3857786 | arxiv = physics/0608014 | s2cid = 59405368 | bibcode = 2006ApJ...653L.153G }}</ref><ref>{{cite web |title=Diamonds from Outer Space: Geologists Discover Origin of Earth's Mysterious Black Diamonds |url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=108270&org=NSF |publisher=[[National Science Foundation]] |date=January 8, 2007 |access-date=October 28, 2007 |archive-date=December 9, 2007 |archive-url=https://web.archive.org/web/20071209203456/http://www.nsf.gov/news/news_summ.jsp?cntn_id=108270&org=NSF |url-status=live }}</ref>

=== Mechanical ===
==== Hardness ====
[[File:Vickers anvil diamons.jpg|thumb|The extreme hardness of diamond in certain orientations makes it useful in materials science, as in this pyramidal diamond embedded in the working surface of a [[Vickers hardness test]]er.]]
Diamond is the hardest material on the [[qualitative property|qualitative]] [[Mohs scale of mineral hardness|Mohs scale]]. To conduct the [[unit of measurement|quantitative]] [[Vickers hardness test]], samples of materials are struck with a pyramid of standardized dimensions using a known force – a diamond crystal is used for the pyramid to permit a wide range of materials to be tested. From the size of the resulting indentation, a Vickers hardness value for the material can be determined. Diamond's great hardness relative to other materials has been known since antiquity, and is the source of its name. This does not mean that it is infinitely hard, indestructible, or unscratchable.<ref>{{Cite web|date=December 16, 2015|title=Diamonds Are Indestructible, Right?|url=https://dominionjewelers.com/diamonds-are-indestructible-right/|access-date=October 31, 2020|website=Dominion Jewelers|language=en-US|archive-date=September 26, 2020|archive-url=https://web.archive.org/web/20200926001227/https://dominionjewelers.com/diamonds-are-indestructible-right/|url-status=live}}</ref> Indeed, diamonds can be scratched by other diamonds<ref>{{cite journal|vauthors=Seal M |title=The abrasion of diamond |journal=Proceedings of the Royal Society A |volume=248 |issue=1254 |date=November 25, 1958 |pages=379–393 |doi=10.1098/rspa.1958.0250|bibcode=1958RSPSA.248..379S }}</ref> and worn down over time even by softer materials, such as vinyl [[phonograph record]]s.<ref>{{cite web |vauthors=Weiler HD |title=The wear and care of records and styli |orig-date=1954 |date=April 13, 2021 |via=Shure Applications Engineering |url=https://service.shure.com/s/article/stylus-wear-and-record-wear?language=en_US |access-date=August 25, 2024 |archive-date=March 26, 2023 |archive-url=https://web.archive.org/web/20230326031532/https://service.shure.com/s/article/stylus-wear-and-record-wear?language=en_US |url-status=live }}</ref>

Diamond hardness depends on its purity, crystalline perfection, and orientation: hardness is higher for flawless, pure crystals oriented to the [[Miller index#Case of cubic structures|<111>]] direction (along the longest diagonal of the cubic diamond lattice).<ref>{{cite book|pages=142–147|url=https://books.google.com/books?id=jtC1mUFZfQcC&pg=PA143|title=Properties, Growth and Applications of Diamond|vauthors=Neves AJ, Nazaré MH|publisher=[[Institution of Engineering and Technology]]|year=2001|isbn=978-0-85296-785-0|access-date=November 9, 2020|archive-date=February 19, 2023|archive-url=https://web.archive.org/web/20230219072829/https://books.google.com/books?id=jtC1mUFZfQcC&pg=PA143|url-status=live}}</ref> Therefore, whereas it might be possible to scratch some diamonds with other materials, such as [[boron nitride]], the hardest diamonds can only be scratched by other diamonds and [[Aggregated diamond nanorod|nanocrystalline diamond aggregates]].

The hardness of diamond contributes to its suitability as a gemstone. Because it can only be scratched by other diamonds, it maintains its polish extremely well. Unlike many other gems, it is well-suited to daily wear because of its resistance to scratching—perhaps contributing to its popularity as the preferred gem in [[engagement ring|engagement]] or [[wedding ring]]s, which are often worn every day.

The hardest natural diamonds mostly originate from the [[Copeton Dam|Copeton]] and [[Bingara]] fields located in the [[New England (Australia)|New England]] area in [[New South Wales]], Australia. These diamonds are generally small, perfect to semiperfect octahedra, and are used to polish other diamonds. Their hardness is associated with the [[crystal growth]] form, which is single-stage crystal growth. Most other diamonds show more evidence of multiple growth stages, which produce inclusions, flaws, and defect planes in the crystal lattice, all of which affect their hardness. It is possible to treat regular diamonds under a combination of high pressure and high temperature to produce diamonds that are harder than the diamonds used in hardness gauges.<ref>{{cite magazine|vauthors=Boser U|title=Diamonds on Demand|url=http://www.smithsonianmag.com/science-nature/diamonds-on-demand.html|magazine=[[Smithsonian (magazine)|Smithsonian]]|volume=39|issue=3|pages=52–59|year=2008|access-date=June 13, 2009|archive-date=March 2, 2012|archive-url=https://web.archive.org/web/20120302163915/http://www.smithsonianmag.com/science-nature/diamonds-on-demand.html|url-status=dead}}</ref>

Diamonds cut glass, but this does not positively identify a diamond because other materials, such as quartz, also lie above glass on the [[Mohs scale]] and can also cut it. Diamonds can scratch other diamonds, but this can result in damage to one or both stones. Hardness tests are infrequently used in practical gemology because of their potentially destructive nature.<ref name=read/> The extreme hardness and high value of diamond means that gems are typically polished slowly, using painstaking traditional techniques and greater attention to detail than is the case with most other gemstones;<ref name="hazen">{{cite book|url=https://books.google.com/books?id=fNJQok6N9_MC&pg=PA7|pages=7–10|title=The diamond makers| vauthors = Hazen RM |publisher=Cambridge University Press|year=1999|isbn=978-0-521-65474-6}}</ref> these tend to result in extremely flat, highly polished facets with exceptionally sharp facet edges. Diamonds also possess an extremely high refractive index and fairly high dispersion. Taken together, these factors affect the overall appearance of a polished diamond and most [[diamantaire]]s still rely upon skilled use of a [[loupe]] (magnifying glass) to identify diamonds "by eye".<ref>{{cite book|url=https://books.google.com/books?id=Jm3FwBiHaI4C&pg=PA37|pages=34–37|title=Synthetic, Imitation and Treated Gemstones| vauthors = O'Donoghue M |publisher=Gulf Professional |year= 1997|isbn=978-0-7506-3173-0}}</ref>

==== Toughness ====
Somewhat related to hardness is another mechanical property ''toughness'', which is a material's ability to resist breakage from forceful impact. The [[toughness]] of natural diamond has been measured as 50–65&nbsp;[[Megapascal|MPa]]·m<sup>1/2</sup>.{{contradiction inline|reason=Unit of toughness as given at the article on toughness is newton-metres per cubic metre, dimensionally equivalent to newtons per square metre i.e. pascals. What is this factor of m^{1/2} doing here? Should we actually be talking about and linking to [[Fracture toughness]] (which unfortunately doesn't have a discussion of units)?|date=October 2023}}<ref>{{cite book| vauthors = Lee J, Novikov NV |title=Innovative superhard materials and sustainable coatings for advanced manufacturing|url=https://books.google.com/books?id=EXGcDYj8HvEC&pg=PA102|page=102|publisher=Springer|year=2005|isbn=978-0-8493-3512-9}}</ref><ref>{{cite book| vauthors = Marinescu ID, Tönshoff HK, Inasaki I |title=Handbook of ceramic grinding and polishing|url=https://books.google.com/books?id=QCvqtRJJ4XwC&pg=PA21|page=21|publisher=William Andrew|year=2000|isbn=978-0-8155-1424-4}}</ref> This value is good compared to other ceramic materials, but poor compared to most engineering materials such as engineering alloys, which typically exhibit toughness over 80{{nbsp}}MPa·m<sup>1/2</sup>. As with any material, the macroscopic geometry of a diamond contributes to its resistance to breakage. Diamond has a [[cleavage plane]] and is therefore more fragile in some orientations than others. [[Diamond cutting|Diamond cutters]] use this attribute to cleave some stones before faceting them.<ref name=harlow/> "Impact toughness" is one of the main indexes to measure the quality of synthetic industrial diamonds.

==== Yield strength ====
Diamond has compressive yield strength of 130–140{{nbsp}}GPa.<ref>{{cite journal | vauthors = Eremets MI, Trojan IA, Gwaze P, Huth J, Boehler R, Blank VD |title=The strength of diamond |journal=Applied Physics Letters |date=October 3, 2005 |volume=87 |issue=14 |pages=141902 |doi=10.1063/1.2061853|bibcode=2005ApPhL..87n1902E}}</ref> This exceptionally high value, along with the hardness and transparency of diamond, are the reasons that [[diamond anvil]] cells are the main tool for high pressure experiments.<ref name=Dubrovinsky>{{cite journal | vauthors = Dubrovinsky L, Dubrovinskaia N, Prakapenka VB, Abakumov AM | title = Implementation of micro-ball nanodiamond anvils for high-pressure studies above 6 Mbar | journal = Nature Communications | volume = 3 | issue = 1 | pages = 1163 | date = October 23, 2012 | pmid = 23093199 | pmc = 3493652 | doi = 10.1038/ncomms2160 | bibcode = 2012NatCo...3.1163D }}</ref> These anvils have reached pressures of {{val|600|u=GPa}}.<ref name="Wogan2012">{{Cite web |vauthors=Wogan T |publisher=Nature Communications |url=http://physicsworld.com/cws/article/news/2012/nov/02/improved-diamond-anvil-cell-allows-higher-pressures-than-ever-before |title=Improved diamond anvil cell allows higher pressures than ever before |work=[[Physics World]] |date=November 2, 2012 |access-date=July 1, 2022 |archive-date=January 2, 2018 |archive-url=https://web.archive.org/web/20180102123446/http://physicsworld.com/cws/article/news/2012/nov/02/improved-diamond-anvil-cell-allows-higher-pressures-than-ever-before |url-status=live }}</ref> Much higher pressures may be possible with [[nanocrystalline material|nanocrystalline]] diamonds.<ref name=Dubrovinsky/><ref name="Wogan2012"/>

==== Elasticity and tensile strength ====
Usually, attempting to deform bulk diamond crystal by tension or bending results in brittle fracture. However, when single crystalline diamond is in the form of micro/nanoscale wires or needles (~100–300{{nbsp}}nanometers in diameter, micrometers long), they can be elastically stretched by as much as 9–10 percent tensile strain without failure,<ref>{{cite journal | vauthors = Dang C, Chou JP, Dai B, Chou CT, Yang Y, Fan R, Lin W, Meng F, Hu A, Zhu J, Han J, Minor AM, Li J, Lu Y | display-authors = 6 | title = Achieving large uniform tensile elasticity in microfabricated diamond | journal = Science | volume = 371 | issue = 6524 | pages = 76–78 | date = January 2021 | pmid = 33384375 | doi = 10.1126/science.abc4174 | doi-access =  | bibcode = 2021Sci...371...76D | s2cid = 229935085 }}</ref> with a maximum local tensile stress of about {{nowrap|89–98 GPa}},<ref>{{cite journal | vauthors = Banerjee A, Bernoulli D, Zhang H, Yuen MF, Liu J, Dong J, Ding F, Lu J, Dao M, Zhang W, Lu Y, Suresh S | display-authors = 6 | title = Ultralarge elastic deformation of nanoscale diamond | journal = Science | volume = 360 | issue = 6386 | pages = 300–302 | date = April 2018 | pmid = 29674589 | doi = 10.1126/science.aar4165 | doi-access =  | bibcode = 2018Sci...360..300B | s2cid = 5047604 }}</ref> very close to the theoretical limit for this material.<ref>{{cite journal | vauthors = LLorca J | title = On the quest for the strongest materials | journal = Science | volume = 360 | issue = 6386 | pages = 264–265 | date = April 2018 | pmid = 29674578 | doi = 10.1126/science.aat5211 | arxiv = 2105.05099 | s2cid = 4986592 | bibcode = 2018Sci...360..264L }}</ref>

=== Electrical conductivity ===
Other specialized applications also exist or are being developed, including use as [[semiconductor]]s: some [[blue diamond]]s are natural semiconductors, in contrast to most diamonds, which are excellent [[insulator (electricity)|electrical insulators]]. The conductivity and blue color originate from boron impurity. Boron substitutes for carbon atoms in the diamond lattice, donating a hole into the [[valence band]].<ref name="boron">{{cite journal | vauthors = Collins AT |title=The Optical and Electronic Properties of Semiconducting Diamond |journal=[[Philosophical Transactions of the Royal Society A]] |volume=342 |pages=233–244 |year=1993 |doi=10.1098/rsta.1993.0017 |issue=1664 |bibcode=1993RSPTA.342..233C |s2cid=202574625}}</ref>

Substantial conductivity is commonly observed in nominally [[doping (semiconductor)|undoped]] diamond grown by [[chemical vapor deposition of diamond|chemical vapor deposition]]. This conductivity is associated with [[hydrogen]]-related species adsorbed at the surface, and it can be removed by [[annealing (metallurgy)|annealing]] or other surface treatments.<ref>{{cite journal | vauthors = Landstrass MI, Ravi KV |title=Resistivity of chemical vapor deposited diamond films |journal=[[Applied Physics Letters]] |volume=55 |pages=975–977 |year=1989 |doi=10.1063/1.101694 |issue=10 |bibcode=1989ApPhL..55..975L}}</ref><ref>{{cite journal | vauthors = Zhang W, Ristein J, Ley L | title = Hydrogen-terminated diamond electrodes. II. Redox activity | journal = Physical Review E | volume = 78 | issue = 4 Pt 1 | pages = 041603 | date = October 2008 | pmid = 18999435 | doi = 10.1103/PhysRevE.78.041603 | bibcode = 2008PhRvE..78d1603Z }}</ref>

Thin needles of diamond can be made to vary their electronic [[band gap]] from the normal 5.6 eV to near zero by selective mechanical deformation.<ref>{{cite journal | vauthors = Shi Z, Dao M, Tsymbalov E, Shapeev A, Li J, Suresh S | title = Metallization of diamond | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 117 | issue = 40 | pages = 24634–24639 | date = October 2020 | pmid = 33020306 | pmc = 7547227 | doi = 10.1073/pnas.2013565117 | doi-access = free | bibcode = 2020PNAS..11724634S }}</ref>

High-purity diamond wafers 5&nbsp;cm in diameter exhibit perfect resistance in one direction and perfect conductance in the other, creating the possibility of using them for quantum data storage. The material contains only 3 parts per million of nitrogen. The diamond was grown on a stepped substrate, which eliminated cracking.<ref>{{Cite web | vauthors = Irving M |date=April 28, 2022 |title=Two-inch diamond wafers could store a billion Blu-Ray's worth of data |url=https://newatlas.com/electronics/2-inch-diamond-wafers-quantum-memory-billion-blu-rays/ |access-date=April 29, 2022 |website=New Atlas |language=en-US}}</ref>

=== Surface property ===
Diamonds are naturally [[lipophilicity|lipophilic]] and [[hydrophobe|hydrophobic]], which means the diamonds' surface cannot be wet by water, but can be easily wet and stuck by oil. This property can be utilized to extract diamonds using oil when making synthetic diamonds. However, when diamond surfaces are chemically modified with certain ions, they are expected to become so [[hydrophile|hydrophilic]] that they can stabilize multiple layers of [[ice|water ice]] at [[human body temperature]].<ref>{{cite journal | vauthors = Wissner-Gross AD, Kaxiras E | title = Diamond stabilization of ice multilayers at human body temperature | journal = Physical Review E | volume = 76 | issue = 2 Pt 1 | pages = 020501 | date = August 2007 | pmid = 17929997 | doi = 10.1103/physreve.76.020501 | url = http://www.alexwg.org/link?url=http%3A%2F%2Fwww.alexwg.org%2Fpublications%2FPhysRevERapidComm_76-020501.pdf | url-status = live | s2cid = 44344503 | bibcode = 2007PhRvE..76b0501W | archive-url = https://web.archive.org/web/20110724214405/http://www.alexwg.org/publications/PhysRevERapidComm_76-020501.pdf | archive-date = July 24, 2011 }}</ref>

The surface of diamonds is partially oxidized. The oxidized surface can be reduced by heat treatment under hydrogen flow. That is to say, this heat treatment partially removes oxygen-containing functional groups. But diamonds (sp<sup>3</sup>C) are unstable against high temperature (above about {{convert|400|C}}) under atmospheric pressure. The structure gradually changes into sp<sup>2</sup>C above this temperature. Thus, diamonds should be reduced below this temperature.<ref>{{cite journal | vauthors = Fujimoto A, Yamada Y, Koinuma M, Sato S | title = Origins of sp(3)C peaks in C1s X-ray Photoelectron Spectra of Carbon Materials | journal = Analytical Chemistry | volume = 88 | issue = 12 | pages = 6110–6114 | date = June 2016 | pmid = 27264720 | doi = 10.1021/acs.analchem.6b01327 | doi-access = free }}</ref>

=== Chemical stability ===
At room temperature, diamonds do not react with any chemical reagents including strong acids and bases.

In an atmosphere of pure oxygen, diamond has an [[ignition point]] that ranges from {{convert|690|C}} to {{convert|840|C}}; smaller crystals tend to burn more easily. It increases in temperature from red to white heat and burns with a pale blue flame, and continues to burn after the source of heat is removed. By contrast, in air the combustion will cease as soon as the heat is removed because the oxygen is diluted with nitrogen. A clear, flawless, transparent diamond is completely converted to carbon dioxide; any impurities will be left as ash.<ref>{{cite book | vauthors = Bauer M |title=Precious Stones | volume = 1 |date=2012 |publisher=Dover Publications |isbn=978-0-486-15125-0 |pages=115–117}}</ref> Heat generated from cutting a diamond will not ignite the diamond,<ref>{{cite web |title=Diamond Care and Cleaning Guide |url=https://www.gia.edu/diamond-care-cleaning |publisher=Gemological Institute of America |access-date=August 1, 2019 |language=en |archive-date=August 1, 2019 |archive-url=https://web.archive.org/web/20190801170616/https://www.gia.edu/diamond-care-cleaning |url-status=live }}</ref> and neither will a cigarette lighter,<ref>{{cite web |vauthors=Jones C |title=Diamonds are Flammable! How to Safeguard Your Jewelry |url=http://www.dmia.net/diamonds-are-flammable/ |website=DMIA |access-date=August 1, 2019 |date=August 27, 2016 |archive-date=August 1, 2019 |archive-url=https://web.archive.org/web/20190801170616/http://www.dmia.net/diamonds-are-flammable/ |url-status=live }}</ref> but house fires and blow torches are hot enough. Jewelers must be careful when molding the metal in a diamond ring.<ref>{{cite web |vauthors=Baird CS |title=Can you light diamond on fire? |url=https://wtamu.edu/~cbaird/sq/2014/03/27/can-you-light-diamond-on-fire/ |website=Science Questions with Surprising Answers |access-date=August 1, 2019 |archive-date=August 1, 2019 |archive-url=https://web.archive.org/web/20190801170618/https://wtamu.edu/~cbaird/sq/2014/03/27/can-you-light-diamond-on-fire/ |url-status=live }}</ref>

Diamond powder of an appropriate grain size (around 50{{nbsp}}microns) burns with a shower of sparks after ignition from a flame. Consequently, [[pyrotechnic composition]]s based on [[synthetic diamond]] powder can be prepared. The resulting sparks are of the usual red-orange color, comparable to charcoal, but show a very linear trajectory which is explained by their high density.<ref>{{cite journal| vauthors = Lederle F, Koch J, Hübner EG |title=Colored Sparks|journal=European Journal of Inorganic Chemistry|date=February 21, 2019|volume=2019|issue=7|pages=928–937|doi=10.1002/ejic.201801300|s2cid=104449284}}</ref> Diamond also reacts with fluorine gas above about {{convert|700|C}}.

=== Color ===
{{Main|Diamond color}}
[[File:National Museum of Natural History Gold Colored Diamonds.JPG|alt=A museum display of jewelry items. Three brooches each consist of a large brown central gem surrounded by many clear small stones. A necklace has a large brown gem at its bottom and its string is all covered with small clear gems. A cluster-shaped decoration contains many brown gems.|upright=1.35|thumb|Brown diamonds at the [[National Museum of Natural History]] in [[Washington, D.C.]]]]
[[File:The Hope Diamond - SIA.jpg|thumb|right|upright=1.35|alt=Picture of a diamond|The most famous colored diamond, the [[Hope Diamond]]]]
Diamond has a wide [[band gap]] of {{val|5.5|ul=eV}} corresponding to the deep [[ultraviolet]] wavelength of 225{{nbsp}}nanometers. This means that pure diamond should transmit visible light and appear as a clear colorless crystal. Colors in diamond originate from lattice defects and impurities. The diamond crystal lattice is exceptionally strong, and only atoms of [[nitrogen]], [[boron]], and [[hydrogen]] can be introduced into diamond during the growth at significant concentrations (up to atomic percents). Transition metals [[nickel]] and [[cobalt]], which are commonly used for growth of synthetic diamond by high-pressure high-temperature techniques, have been detected in diamond as individual atoms; the maximum concentration is 0.01% for nickel<ref>{{cite journal | vauthors = Collins AT, Kanda H, Isoya J, Ammerlaan CA, Van Wyk JA |title=Correlation between optical absorption and EPR in high-pressure diamond grown from a nickel solvent catalyst |journal=Diamond and Related Materials |volume=7 |pages=333–338 |year=1998 |doi=10.1016/S0925-9635(97)00270-7 |issue=2–5 |bibcode=1998DRM.....7..333C }}</ref> and even less for cobalt. Virtually any element can be introduced to diamond by ion implantation.<ref>{{cite journal |doi=10.1103/PhysRevB.61.12909 |title=Vibronic spectra of impurity-related optical centers in diamond |year=2000 | vauthors = Zaitsev AM |journal=Physical Review B |volume=61 |pages=12909–12922 |issue=19 |bibcode=2000PhRvB..6112909Z}}</ref>

Nitrogen is by far the most common impurity found in gem diamonds and is responsible for the yellow and brown color in diamonds. Boron is responsible for the blue color.<ref>{{cite journal| vauthors = Walker J |title=Optical absorption and luminescence in diamond|journal=Reports on Progress in Physics|volume=42|pages=1605–1659|year=1979|doi=10.1088/0034-4885/42/10/001|issue=10|bibcode=1979RPPh...42.1605W|url=http://accreditedgemologists.org/lightingtaskforce/OpticalAbsorptionand.pdf |archive-url=https://web.archive.org/web/20150906142645/http://accreditedgemologists.org/lightingtaskforce/OpticalAbsorptionand.pdf |archive-date=September 6, 2015 |url-status=live|citeseerx=10.1.1.467.443|s2cid=250857323 }}</ref> Color in diamond has two additional sources: irradiation (usually by alpha particles), that causes the color in green diamonds, and [[plastic deformation]] of the diamond crystal lattice. Plastic deformation is the cause of color in some brown<ref>{{cite journal| vauthors = Hounsome LS, Jones R, Shaw MJ, Briddon PR, Öberg S, Briddon P, Öberg S |title=Origin of brown coloration in diamond |journal=[[Physical Review B]] |volume=73 |page=125203|year=2006|doi=10.1103/PhysRevB.73.125203 |issue=12|bibcode=2006PhRvB..73l5203H}}</ref> and perhaps pink and red diamonds.<ref>{{cite book| vauthors = Wise RW |title=Secrets Of The Gem Trade, The Connoisseur's Guide To Precious Gemstones|publisher=Brunswick House Press|pages=223–224|year=2001|isbn=978-0-9728223-8-1}}</ref> In order of increasing rarity, yellow diamond is followed by brown, colorless, then by blue, green, black, pink, orange, purple, and red.<ref name=harlow/> "Black", or [[carbonado]], diamonds are not truly black, but rather contain numerous dark inclusions that give the gems their dark appearance. Colored diamonds contain impurities or structural defects that cause the coloration, while pure or nearly pure diamonds are transparent and colorless. Most diamond impurities replace a carbon atom in the [[crystal lattice]], known as a [[carbon flaw]]. The most common impurity, nitrogen, causes a slight to intense yellow coloration depending upon the type and concentration of nitrogen present.<ref name=harlow/> The [[Gemological Institute of America]] (GIA) classifies low saturation yellow and brown diamonds as diamonds in the ''normal color range'', and applies a grading scale from "D" (colorless) to "Z" (light yellow). Yellow diamonds of high color saturation or a different color, such as pink or blue, are called ''fancy colored'' diamonds and fall under a different grading scale.<ref name=harlow/>

In 2008, the [[Wittelsbach Diamond]], a {{convert|35.56|carat|g|adj=on}} [[blue diamond]] once belonging to the King of Spain, fetched over US$24&nbsp;million at a Christie's auction.<ref>{{cite news |vauthors=Khan U |title=Blue-grey diamond belonging to King of Spain has sold for record 16.3{{nbsp}}GBP |url=https://www.telegraph.co.uk/culture/3703861/Blue-grey-diamond-belonging-to-King-of-Spain-has-sold-for-record-16.3m.html |work=[[The Daily Telegraph]] |location=London |date=December 10, 2008 |access-date=March 31, 2010 |archive-date=February 7, 2009 |archive-url=https://web.archive.org/web/20090207212758/http://www.telegraph.co.uk/culture/3703861/Blue-grey-diamond-belonging-to-King-of-Spain-has-sold-for-record-16.3m.html |url-status=live }}</ref> In May 2009, a {{convert|7.03|carat|g|adj=on}} [[blue diamond]] fetched the highest price per carat ever paid for a diamond when it was sold at auction for 10.5&nbsp;million Swiss francs (6.97&nbsp;million euros, or US$9.5&nbsp;million at the time).<ref>{{cite news|vauthors=Nebehay S|title=Rare blue diamond sells for record $9.5&nbsp;million|url=https://www.reuters.com/article/artsNews/idUSTRE54B6O020090512|work=Reuters|date=May 12, 2009|access-date=May 13, 2009|archive-date=May 16, 2009|archive-url=https://web.archive.org/web/20090516234031/http://www.reuters.com/article/artsNews/idUSTRE54B6O020090512|url-status=live}}</ref> That record was, however, beaten the same year: a {{convert|5|carat|g|adj=on}} vivid pink diamond was sold for US$10.8&nbsp;million in Hong Kong on December 1, 2009.<ref>{{cite news|url=https://www.reuters.com/article/idUSTRE5B02P620091201|title=Vivid pink diamond sells for record $10.8&nbsp;million|work=Reuters|date=December 1, 2009|vauthors=Pomfret J|access-date=July 1, 2017|archive-date=December 2, 2020|archive-url=https://web.archive.org/web/20201202103252/https://www.reuters.com/article/idUSTRE5B02P620091201|url-status=live}}</ref>

=== Clarity ===
Clarity is one of the 4C's (color, clarity, cut and carat weight) that helps in identifying the quality of diamonds. The [[Gemological Institute of America]] (GIA) developed 11 clarity scales to decide the quality of a diamond for its sale value. The GIA clarity scale spans from Flawless (FL) to included (I) having internally flawless (IF), very, very slightly included (VVS), very slightly included (VS) and slightly included (SI) in between. Impurities in natural diamonds are due to the presence of natural minerals and oxides. The clarity scale grades the diamond based on the color, size, location of impurity and quantity of clarity visible under 10x magnification.<ref>{{cite journal | vauthors = Cowing MD |year=2014 |title=Objective ciamond clarity grading |journal=Journal of Gemmology |volume=34 |number=4 |pages=316–332 |doi=10.15506/JoG.2014.34.4.316 |bibcode=2014JGem...34..316C |url=https://acagemlab.com/wp-content/uploads/2019/01/JoG2014_34_4_Cowing_Obj_Diamond_Clarity-1.pdf |archive-url=https://web.archive.org/web/20210418052002/https://acagemlab.com/wp-content/uploads/2019/01/JoG2014_34_4_Cowing_Obj_Diamond_Clarity-1.pdf |archive-date=April 18, 2021 |url-status=live |access-date=September 19, 2021}}</ref> Inclusions in diamond can be extracted by optical methods. The process is to take pre-enhancement images, identifying the inclusion removal part and finally removing the diamond facets and noises.<ref>{{cite journal | vauthors = Wang W, Cai L | title = Inclusion extraction from diamond clarity images based on the analysis of diamond optical properties | journal = Optics Express | volume = 27 | issue = 19 | pages = 27242–27255 | date = September 2019 | pmid = 31674589 | doi = 10.1364/OE.27.027242 | bibcode = 2019OExpr..2727242W | s2cid = 203141270 | doi-access = free }}</ref>

=== Fluorescence ===
[[File:Rough diamonds - necklace in UV and normal light B - composite.jpg|thumb|Necklace of rough light brown diamonds under [[Blacklight|UV light]] (top) and normal light (bottom)]]
Between 25% and 35% of natural diamonds exhibit some degree of fluorescence when examined under invisible long-wave ultraviolet light or higher energy radiation sources such as X-rays and lasers.<ref>{{Cite web |date=March 27, 2018 |title=Fact Checking Diamond Fluorescence: 11 Myths Dispelled |url=https://4cs.gia.edu/en-us/blog/fact-checking-diamond-fluorescence-myths-dispelled/ |access-date=June 6, 2022 |website=GIA 4Cs |language=en-US |archive-date=March 24, 2022 |archive-url=https://web.archive.org/web/20220324052612/https://4cs.gia.edu/en-us/blog/fact-checking-diamond-fluorescence-myths-dispelled/ |url-status=live }}</ref> Incandescent lighting will not cause a diamond to fluoresce. Diamonds can fluoresce in a variety of colors including blue (most common), orange, yellow, white, green and very rarely red and purple. Although the causes are not well understood, variations in the atomic structure, such as the number of nitrogen atoms present are thought to contribute to the phenomenon.

=== Thermal conductivity ===
Diamonds can be identified by their high thermal conductivity (900–{{val|2320|u=W·m{{Sup|−1}}·K{{Sup|−1}}}}).<ref>{{cite journal | vauthors = Wei L, Kuo PK, Thomas RL, Anthony TR, Banholzer WF | title = Thermal conductivity of isotopically modified single crystal diamond | journal = Physical Review Letters | volume = 70 | issue = 24 | pages = 3764–3767 | date = June 1993 | pmid = 10053956 | doi = 10.1103/PhysRevLett.70.3764 | bibcode = 1993PhRvL..70.3764W }}</ref> Their high [[refractive index]] is also indicative, but other materials have similar refractivity.