Luster
Luster is the quality of reflected light from the surface of a gem or mineral and may vary somewhat in a single crystal. For example, a crystal may have a vitreous luster on the surfaces parallel to its cleavage and have a pearly or silky luster perpendicular to the cleavage due to its fibrous nature. Gypsum is an example of this phenomenon. Luster is not a very useful diagnostic property in identifying such minerals.
Luster is divided into two basic types: metallic and non-metallic. There are also intermediate types called sub-metallic. Any gem or mineral that does not have a metallic appearance is described as non-metallic. The luster of gems and minerals is described in the following terms (listed in alphabetical order):
- Adamantine – hard, steely brilliance like the reflection from a diamond (high index of refraction)
- Chatoyant – Numerous hair-like inclusions aligned to produce “Catseye” effect
- Dull – Completly dull, eg. clays
- Earthy (Dull) – Completly dull, eg. clays
- Greasy – appears to be covered in oil or grease
- Metallic – Specular reflection, eg. Pyrite
- Metallic-Dull – Has a dull metallic luster
- Pearly – Formed by numerous partly-developed cleavages, eg. Pearls
- Resinous – Luster of Resin, eg. Amber
- Schiller – Caused by numerous platy inclusions, eg. Sunstone
- Silky – Noticeable, fibrous, shiny direction, eg. Satin Spar
- Sub-Metallic – Almost metallic reflection, eg. Cuprite
- Sub-Adamantine – Not quite adamantine in luster
- Vitreous – Luster of broken glass (most gem minerals fall in this type), eg. Quartz
- Waxy – Somewhat dull luster with a hint of shininess, eg. Turquoise, Variscite
