Lustre (Commonwealth English) or luster (American English; see spelling differences) is the way light interacts with the surface of a crystal, 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 (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
Adamantine lustre
Adamantine minerals possess a superlative lustre, which is most notably seen in
diamond.
Such minerals are transparent or translucent, and have a high
refractive index (of 1.9 or more).
Minerals with a true adamantine lustre are uncommon, with examples including
cerussite,
zircon, and
cubic zirconia.
Minerals with a lesser (but still relatively high) degree of lustre are referred to as subadamantine, with some examples being garnet and corundum.
Dull lustre
Dull (or
earthy) minerals exhibit little to no lustre, due to coarse granulations which scatter light in all directions, approximating a Lambertian reflector. An example is
kaolinite.
A distinction is sometimes drawn between dull minerals and earthy minerals,
Rosie Hankin (1998). 186155480X, Quintet Publishing. 186155480X
with the latter being coarser, and having even less lustre.
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.
Many minerals with a greasy lustre also feel greasy to the touch.
Metallic lustre
Metallic (or
splendent) minerals have the lustre of polished metal, and with
will work as a reflective surface. Examples include
galena,
and
magnetite.
Pearly lustre
Pearly minerals consist of thin transparent co-planar sheets. Light reflecting from these layers gives them a lustre reminiscent of
.
Such minerals possess perfect cleavage, with examples including
muscovite and
stilbite.
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 fossilised resin.
Silky lustre
Silky minerals have a parallel arrangement of extremely fine fibres,
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.
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,
such as
sphalerite,
cinnabar,
anthracite, and
cuprite.
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,
and occurs in transparent or translucent minerals with relatively low refractive indices.
Common examples include
calcite,
quartz,
topaz,
beryl,
tourmaline and
fluorite, among others.
Waxy lustre
Waxy minerals have a lustre resembling
wax. Examples include
jade and
chalcedony.
Optical phenomena
Asterism
Asterism is the display of a star-shaped luminous area. It is seen in some
and
ruby, where it is caused by impurities of
rutile.
It can also occur in
garnet,
diopside and
spinel.
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, chrome-bearing
fuchsite makes for a green stone and various
make for a red stone.
Chatoyancy
Chatoyancy minerals display luminous bands, which appear to move as the specimen is rotated. Such minerals are composed of parallel fibres (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 cymophane, but the effect may also occur in other minerals such as aquamarine, moonstone and
tourmaline.
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.
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.
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 and
labradorite and is very similar to
adularescence and
aventurescence.
