Luster

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Lustre is a surface gloss (caused by reflection), which depends to a large degree on:

  • the refractive index.
  • the abillity to take polish of a gemstone.

The better the polish of the gemstone, the better the lustre.
Since hardness defines what polish a stone can take, you can state:

  • The amount and quality of the reflected light, which is called lustre, is the highest for gemstones with a high refractive index, which also can take an extremely good polish. In practise, these are usually the stones with the greatest hardness.

However, there is no direct relationship between hardness and lustre.
For instance: Sphalerite, while having and RI of 2.39, only has a hardness of 3.5 on Moh's scale, yet has a sub-adamantine lustre.

The following terms are used to describe the lustre of various gemstones:

  1. Adamantine: the type of lustre described by Diamond.
  2. Sub-adamantine: gemstones having a high RI (but lower than Diamond), like Zircon and Sphene.
  3. Vitreous: gemstones whose refractive indices fall within the range of middle values, for instance Emerald, Ruby, Spinel and most other transparent gemstones. This lustre is sometimes described as "glass-like".
  4. Resinous: gem materials that are soft and that have low refractive indices, like Amber and Opal.
  5. Waxy: an almost matt surface like exhibited by Turquoise and Jadeite.
  6. Silky: certain fibrous materials such as Gypsum and Malachite.
  7. Pearly: the lustre seen in Pearl, Moonstone and Talc.
  8. Metallic: the very high lustre shown by metals such as gold and silver, and by minerals 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 lustre, yet this never was put in practise. Probably because of the more romantic connotations of the verbal descriptions.