Lustre (British English) orluster (American English;see spelling differences) is the waylight interacts with the surface of acrystal,rock, ormineral. The word traces its origins back to theLatinlux, meaning "light", and generally implies radiance, gloss, or brilliance.
A range of terms are used to describe lustre, such asearthy,metallic,greasy, andsilky. Similarly, the termvitreous (derived from the Latin forglass,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 asasterism (the display of a star-shaped luminous area) orchatoyancy (the display of luminous bands, which appear to move as the specimen is rotated). A list of such phenomena is given below.
Adamantine minerals possess a superlative[clarification needed] lustre, which is most notably seen indiamond.[1] Such minerals are transparent or translucent, and have a highrefractive index (of 1.9 or more).[2] Minerals with a true adamantine lustre are uncommon, with examples includingcerussite,zircon, andcubic zirconia.[2]
Minerals with a lesser (but still relatively high) degree of lustre are referred to assubadamantine, with some examples beinggarnet andcorundum.[1]
Dull (orearthy) minerals exhibit little to no lustre, due to coarse granulations which scatter light in all directions, approximating aLambertian reflector. An example iskaolinite.[3] A distinction is sometimes drawn between dull minerals and earthy minerals,[4] with the latter being coarser, and having even less lustre.
Greasy minerals resemble fat or grease. A greasy lustre often occurs in minerals containing a great abundance of microscopic inclusions, with examples includingopal andcordierite,jadeite.[2] Many minerals with a greasy lustre also feel greasy to the touch.[5]
Metallic (orsplendent) minerals have the lustre of polished metal, and withideal surfaces will work as areflective surface. Examples includegalena,[6]pyrite[7] andmagnetite.[8]
Pearly minerals consist of thin transparent co-planar sheets. Light reflecting from these layers gives them a lustre reminiscent ofpearls.[9] Such minerals possess perfectcleavage, with examples includingmuscovite andstilbite.[2]
Resinous minerals have the appearance ofresin,chewing gum or (smooth-surfaced) plastic. A principal example isamber, which is a form of fossilized resin.[10]
Silky minerals have a parallel arrangement of extremely fine fibres,[2] giving them a lustre reminiscent ofsilk. Examples includeasbestos,ulexite and thesatin spar variety ofgypsum. Afibrous lustre is similar, but has a coarser texture.
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,[2] such assphalerite,cinnabar,anthracite, andcuprite.
Vitreous minerals have the lustre ofglass. (The term is derived from the Latin for glass,vitrum.) This type of lustre is one of the most commonly seen,[9] and occurs in transparent or translucent minerals with relatively low refractive indices.[2] Common examples includecalcite,quartz,topaz,beryl,tourmaline andfluorite, among others.
Waxy minerals have a lustre resemblingwax. Examples includejade[11] andchalcedony.[12]
Asterism is the display of a star-shaped luminous area. It is seen in somesapphires andrubies, where it is caused by impurities ofrutile.[12][13] It can also occur ingarnet,diopside andspinel.
Aventurescence (oraventurization) is a reflectance effect like that ofglitter. 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. Inaventurine quartz, chrome-bearingfuchsite makes for a green stone and variousiron oxides make for a red stone.[12]
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 aretiger's eye andcymophane, but the effect may also occur in other minerals such asaquamarine,moonstone andtourmaline.
Colour change is most commonly found in alexandrite, a variety ofchrysoberyl 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 strongpleochroism. 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 theUral 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.
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 preciousopal.[14]
Schiller (German, literally "shimmer"), is the metalliciridescence originating from below the surface of a stone that occurs when light is reflected between layers of minerals. It is seen inmoonstone andlabradorite and is very similar toadularescence andaventurescence.[15]
{{cite book}}: CS1 maint: multiple names: authors list (link)
