Adamantine. Silky. Greasy? These are a few of the evocative terms that can be used to describe gemstone luster. Gemstone luster simply means how a gemstone looks when it reflects light. For example, the brilliant mirror-like appearance of a diamond is called adamantine.
Some gems, like ulexite, show fine parallel threads that look like the texture of fabric and are said to have silky luster.
Other gems may appear oily or fatty in the light, like green serpentine. This type of luster is referred to as greasy.
Although gemstone luster is one of the most obvious characteristics of a stone, assigning luster is somewhat subjective. You might find some gemstones described as possessing a blend of two types of luster or as falling within a range. For example, serpentines may also be called resinous, pearly, or waxy. Some gemstones may show a different luster depending on which side is facing the light.
What is the Difference Between Luster, Polish Luster, and Fracture Luster?
The condition of a gem may also affect its luster. A gemstone can have a polish luster (its appearance when polished) that varies greatly from its base luster. A gemstone’s fracture luster is how its fractures look when they reflect light. (Fractures are breaks that are not along a gemstone’s cleavage or regular plane surfaces).
In addition to adamantine, silky, and greasy, here are additional terms for luster commonly used to characterize gemstones.
A dull luster simply means a gemstone reflects little light, such a rhodonite.
Metallic luster, a reflective metal-like appearance, is a term not usually used for gemstones. Hematite is an exception. It has a striking metallic sheen and has been used for carving cameos and making beads.