Lustre (mineralogy)

The word traces its origins back to the Latin lux, meaning "light", and generally implies radiance, gloss, or brilliance.

[2] Minerals with a true adamantine lustre are uncommon, with examples including cerussite, zircon, and cubic zirconia.

[1] Dull (or earthy) minerals exhibit little to no lustre, due to coarse granulations which scatter light in all directions, approximating a Lambertian reflector.

A greasy lustre often occurs in minerals containing a great abundance of microscopic inclusions, with examples including opal and cordierite, jadeite.

[10] Silky minerals have a parallel arrangement of extremely fine fibres,[2] giving them a lustre reminiscent of silk.

A submetallic lustre often occurs in near-opaque minerals with very high refractive indices,[2] such as sphalerite, cinnabar, anthracite, and cuprite.

This type of lustre is one of the most commonly seen,[9] and occurs in transparent or translucent minerals with relatively low refractive indices.

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.

The gem results from small-scale replacement of aluminium by chromium oxide, which is responsible for alexandrite's characteristic green to red colour change.

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.

[14]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.