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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.
Non-metallic gems and minerals are 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
- Pearly
- Formed by numerous partly-developed cleavages, eg.
Pearls
- Resinous
- Luster of Resin, eg. Amber
- Schiller
- Caused by numerous platy inclusions or
separations, eg. Sunstone
- Silky
- Noticeable, fibrous, shiney 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
- Fairly dull luster, eg. wax
Terms
of luster for metallic gems:
- Metallic
- Specular reflection, eg. Pyrite
- Metallic-Dull
- Has a dull metallic luster
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