Editing Diamond (section)

=== 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>