Luster (or lustre) is a surface gloss (caused by reflection), which depends to a large degree on:
- the refractive index.
- the ability to take polish of a gemstone.
The better the polish of the gemstone, the better the luster.
Since hardness defines what polish a stone can take, you can state:
- The amount and quality of the reflected light (which is called luster) is the highest for gemstones with a high refractive index, which also take an extremely good polish. In practice, these are usually the stones with the greatest hardness.
However, there is no direct relationship between hardness and luster. For instance: Sphalerite, while having an RI of 2.39, only has a hardness of 3.5 on Moh's scale, yet has a sub-adamantine (not quite diamond-like) luster.
The following terms are used to describe the luster of various gemstones:
- Adamantine: the type of luster described by Diamond.
- Sub-adamantine: gemstones having a high RI (but lower than Diamond), like Zircon and Sphene.
- Vitreous: gemstones whose refractive indices fall within the range of middle values such as Emerald, Ruby, Spinel and most other transparent gemstones. Vitreous luster is sometimes described as "glass-like".
- Resinous: gem materials that are soft and have low refractive indices, like Amber and Opal.
- Waxy: an almost matt surface like that exhibited by Turquoise and Jadeite.
- Silky: certain fibrous materials such as Gypsum and Malachite.
- Pearly: the luster seen in Pearl, Moonstone, and Talc.
- Metallic: the very high luster shown by metals such as gold and silver, and by minerals such as polished Hematite.
According to Robert Webster in "Gems: Their sources, descriptions, and identification", Dr. Hanneman proposed to give a numeric value to the types of luster. This never was put into practice, probably because of the more romantic connotations of the verbal descriptions.