Editing Lustre (mineralogy)

{{Short description|Manner in which light interacts with a crystal, rock, or mineral's surface}}
{{Use British English|date = September 2019}}

'''Lustre''' ([[English in the Commonwealth of Nations|Commonwealth English]]) or '''luster''' ([[American English]]; [[American and British English spelling differences#-re, -er|see spelling differences]]) is the way [[light]] interacts with the surface of a [[crystal]], [[rock (geology)|rock]], or [[mineral]]. The word traces its origins back to the [[Latin]] ''lux'', meaning "light", and generally implies radiance, gloss, or brilliance. 

A range of terms are used to describe lustre, such as ''earthy'', ''metallic'', ''greasy'', and ''silky''.  Similarly, the term ''vitreous'' (derived from the Latin for [[glass]], ''vitrum'') refers to a glassy lustre.  A list of these terms is given below.

Lustre varies over a wide continuum, and so there are no rigid boundaries between the different types of lustre.  (For this reason, different sources can often describe the same mineral differently.  This ambiguity is further complicated by lustre's ability to vary widely within a particular mineral species).  The terms are frequently combined to describe intermediate types of lustre (for example, a "vitreous greasy" lustre).

Some minerals exhibit unusual optical phenomena, such as [[Asterism (gemmology)|asterism]] (the display of a star-shaped luminous area) or [[chatoyancy]] (the display of luminous bands, which appear to move as the specimen is rotated).  A list of such phenomena is given below.

== Common terms ==
[[Image:Brillanten.jpg|thumb|upright|[[Diamond cut|Cut diamonds]]]]
=== Adamantine lustre ===
'''Adamantine''' minerals possess a superlative{{Clarify|date=December 2022}} lustre, which is most notably seen in [[diamond]].<ref name="GRG">{{cite book|publisher=[[Gemological Institute of America]]|title=GIA Gem Reference Guide|year=1995|isbn=0-87311-019-6}}</ref> Such minerals are transparent or translucent, and have a high [[refractive index]] (of 1.9 or more).<ref name="duda_rejl">{{cite book| title = Minerals of the World|author1=Duda, Rudolf  |author2=Rejl, Lubos |name-list-style=amp | publisher = Arch Cape Press| year = 1990| isbn = 0-517-68030-0}}</ref>  Minerals with a true adamantine lustre are uncommon, with examples including [[cerussite]], [[zircon]], and [[cubic zirconia]].<ref name="duda_rejl" />

Minerals with a lesser (but still relatively high) degree of lustre are referred to as '''subadamantine''', with some examples being [[garnet]] and [[corundum]].<ref name="GRG"/>
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[[Image:KaolinUSGOV.jpg|thumb|upright|[[Kaolinite]]]]
=== Dull lustre ===
'''Dull''' (or '''earthy''') minerals exhibit little to no lustre, due to coarse granulations which scatter light in all directions, approximating a [[Lambertian reflectance|Lambertian reflector]].  An example is [[kaolinite]].<ref name="webmineral_kaolinite">{{cite web| url = http://www.webmineral.com/data/Kaolinite.shtml| title = Webmineral: Kaolinite Mineral Data| access-date=2008-06-21}}</ref>  A distinction is sometimes drawn between dull minerals and earthy minerals,<ref>{{cite book| title = Rocks, Crystals & Minerals| last=Hankin| first=Rosie| publisher = Quintet Publishing| isbn = 1-86155-480-X| year = 1998 }}</ref> with the latter being coarser, and having even less lustre.
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[[Image:Moos-Opal1.jpg|thumb|upright|Moss [[opal]]]]
=== Greasy lustre ===
'''Greasy''' minerals resemble fat or grease.  A greasy lustre often occurs in minerals containing a great abundance of microscopic inclusions, with examples including [[opal]] and [[cordierite]], [[jadeite]].<ref name="duda_rejl" />  Many minerals with a greasy lustre also feel greasy to the touch.<ref name="GO_340">{{cite web| url = http://www.emporia.edu/earthsci/amber/go340/visual.htm| title = Emporia State University: GO 340 Gemstones & Gemology: Visual Properties| access-date = 2008-06-19| archive-url = https://web.archive.org/web/20110612080930/http://www.emporia.edu/earthsci/amber/go340/visual.htm| archive-date = 2011-06-12| url-status = dead}}</ref>
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[[File:Pyrite 3.jpg|thumb|upright|[[Pyrite]]]]
=== Metallic lustre ===
'''Metallic''' (or '''splendent''') minerals have the lustre of polished metal, and with [[ideal surface]]s will work as a [[reflection (physics)|reflective surface]]. Examples include [[galena]],<ref name="webmineral_galena">{{cite web| url = http://webmineral.com/data/Galena.shtml| title = Webmineral: Galena Mineral Data| access-date=2008-07-05}}</ref> [[pyrite]]<ref name="webmineral_pyrite">{{cite web| url = http://webmineral.com/data/Pyrite.shtml| title = Webmineral: Pyrite Mineral Data| access-date=2008-07-05}}</ref> and [[magnetite]].<ref name="webmineral_magnetite">{{cite web| url = http://webmineral.com/data/Magnetite.shtml| title = Webmineral: Magnetite Mineral Data| access-date=2008-07-05}}</ref>
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[[Image:Mineral Mica GDFL006.JPG|thumb|upright|[[Muscovite]]]]
=== Pearly lustre ===
'''Pearly''' minerals consist of thin transparent co-planar sheets.  Light reflecting from these layers gives them a lustre reminiscent of [[pearl]]s.<ref name="rockcollector" />  Such minerals possess perfect [[Cleavage (crystal)|cleavage]], with examples including [[muscovite]] and [[stilbite]].<ref name="duda_rejl" />
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[[Image:Amber hg.jpg|thumb|upright|[[Amber]]]]

=== Resinous lustre ===
'''Resinous''' minerals have the appearance of [[resin]], [[chewing gum]] or (smooth-surfaced) plastic. A principal example is [[amber]], which is a form of fossilized resin.<ref name="webmineral_amber">{{cite web| url = http://webmineral.com/data/Amber.shtml| title = Webmineral: Amber Mineral Data| access-date=2008-06-21}}</ref>
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[[Image:Selenite Gips Marienglas.jpg|thumb|upright|[[Satin spar]] variety of [[gypsum]]]]
=== Silky lustre ===
'''Silky''' minerals have a parallel arrangement of extremely fine fibres,<ref name="duda_rejl" /> giving them a lustre reminiscent of [[silk]].  Examples include [[asbestos]], [[ulexite]] and the [[satin spar]] variety of [[gypsum]].  A '''fibrous''' lustre is similar, but has a coarser texture.
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[[Image:Sphalerite4.jpg|thumb|upright|[[Sphalerite]]]]
=== Submetallic lustre ===
'''Submetallic''' minerals have similar lustre to metal, but are duller and less reflective.  A submetallic lustre often occurs in near-opaque minerals with very high refractive indices,<ref name="duda_rejl" /> such as [[sphalerite]], [[cinnabar]], [[anthracite]], and [[cuprite]].
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[[Image:Quartz Brésil.jpg|thumb|upright|[[Quartz]]]]
=== Vitreous lustre ===
'''Vitreous''' minerals have the lustre of [[glass]]. (The term is derived from the Latin for glass, ''vitrum''.) This type of lustre is one of the most commonly seen,<ref name="rockcollector">{{cite web| url = http://www.rockcollector.co.uk/opticalprop.htm| title = Optical properties of Rocks and Minerals| access-date=2008-06-01}}</ref> and occurs in transparent or translucent minerals with relatively low refractive indices.<ref name="duda_rejl" />  Common examples include [[calcite]], [[quartz]], [[topaz]], [[beryl]], [[tourmaline]] and [[fluorite]], among others.
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[[Image:Jadestein.jpg|thumb|upright=0.55|[[Jade]]]]
=== Waxy lustre ===
'''Waxy''' minerals have a lustre resembling [[wax]].  Examples include [[jade]]<ref name="emporia_jade">{{cite web| url =http://www.emporia.edu/earthsci/amber/go340/jade.htm| title =Emporia State University: GO 340 Gemstones & Gemology: Jade| access-date =2008-07-14| archive-url =https://web.archive.org/web/20110612081228/http://www.emporia.edu/earthsci/amber/go340/jade.htm| archive-date =2011-06-12| url-status =dead}}</ref> and [[chalcedony]].<ref name="rock_and_gem">{{cite book |last=Bonewitz |first=Ronald Louis |title=Rock and Gem |year=2005 |isbn=0-7513-4400-1 |pages=152&ndash;153 |publisher=[[Dorling Kindersley]]}}</ref>
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==Optical phenomena==
[[Image:Star-Saphire.jpg|thumb|upright=0.5|Sapphire]]
=== Asterism ===
'''[[Asterism (gemmology)|Asterism]]''' is the display of a star-shaped luminous area.  It is seen in some [[sapphire]]s and [[ruby|rubies]], where it is caused by impurities of [[rutile]].<ref name="rock_and_gem" /><ref name="BuildingBlocks451-3">{{cite book |last=Emsley |first=John |title=Nature's Building Blocks: An A-Z Guide to the Elements |year=2001 |isbn=0-19-850341-5 |pages=451&ndash;53 |publisher=[[Oxford University Press]]}}</ref>  It can also occur in [[garnet]], [[diopside]] and [[spinel]].
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[[Image:Aventurine.jpg|thumb|upright|[[Aventurine]]]]
=== Aventurescence ===
'''[[Aventurescence]]''' (or '''aventurization''') is a reflectance effect like that of [[glitter]].  It arises from minute, preferentially oriented mineral platelets within the material. These platelets are so numerous that they also influence the material's body colour. In [[aventurine|aventurine quartz]], chrome-bearing [[fuchsite]] makes for a green stone and various [[iron oxide]]s make for a red stone.<ref name="rock_and_gem" />
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[[Image:Tigers-Eye.jpg|thumb|upright=0.6|[[Tiger's eye]]]]
=== Chatoyancy ===
'''[[Chatoyancy|Chatoyant]]''' minerals display luminous bands, which appear to move as the specimen is rotated.  Such minerals are composed of parallel fibers (or contain fibrous voids or inclusions), which reflect light into a direction perpendicular to their orientation, thus forming narrow bands of light.  The most famous examples are [[tiger's eye]] and [[Chrysoberyl#Cymophane|cymophane]], but the effect may also occur in other minerals such as [[Beryl#Aquamarine and maxixe|aquamarine]], [[Moonstone (gemstone)|moonstone]] and [[tourmaline]].
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[[Image:Alexandrite 26.75cts.jpg|thumb|upright|[[Alexandrite]]]]
=== Colour change ===
'''Colour change''' is most commonly found in alexandrite, a variety of [[chrysoberyl]] gemstones. Other gems also occur in colour-change varieties, including (but not limited to) [[sapphire]], [[garnet]], [[spinel]]. Alexandrite displays a colour change dependent upon light, along with strong [[pleochroism]]. The gem results from small-scale replacement of aluminium by chromium oxide, which is responsible for alexandrite's characteristic green to red colour change. Alexandrite from the [[Ural Mountains]] in Russia is green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light. The optimum or "ideal" colour change would be fine emerald green to fine purplish red, but this is rare.
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=== Iridescence ===
'''Iridescence''' is the 'play' or 'fire' of rainbow-coloured light caused by very thin regular structures or layers beneath the surface of a gemstone. Similar to a thin film of oil on water, these layers interfere with the rays of reflected light, reinforcing some colours and cancelling others. Iridescence is seen at its best in precious [[opal]].<ref>{{Cite book|title=Gemmology|last=G.|first=Read, Peter|date=2008|publisher=N.A.G|isbn=9780719803611|edition= 3rd|location=London|oclc=226280870}}</ref>{{clear}}

[[Image:Labradoryt, Madagaskar.JPG|thumb|upright|[[Labradorite]]]]
=== Schiller ===
'''Schiller''' (German, literally "shimmer"), is the metallic [[iridescence]] originating from below the surface of a stone that occurs when light is reflected between layers of minerals. It is seen in [[Moonstone (gemstone)|moonstone]] and [[labradorite]] and is very similar to [[adularescence]] and [[aventurescence]].<ref>{{cite book|author=Shipley, Robert M. |year=2007|title=Dictionary of gems and gemology| publisher= Read Books|page=93|isbn=978-0-87311-007-5}}</ref>
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==References==

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