The quality and quantity of light reflected from the surface of a gemstone. Luster depends on the refractive index and the polish or surface condition of the gemstone. The ability to take a good polish is much related to the hardness of a gemstone, hence gemstones with a large refractive index as well as a relative high hardness have the highest luster. The mineral sphalerite is one of the exemptions to this rule as it has a low hardness of 3.5 on Moh's scale, but still has a subadamantine luster.

Metallic Metallic luster is the highest luster. Found on gems with a refractive index of 2.5 or higher. Equivalent to the polish on a metal object. Example: Hematite
Submetallic Submetallic luster is slightly less lustrous than metallic.
Adamantine Adamantine luster is the luster exhibited by Diamonds and gems with a Refractive Index of 1.9 - 2.5.
Subadamantine Subadamantine luster is slightly less than that of a diamond. Example: Zircon
Vitreous Vitreous is defined as a glass-like luster. Gems with a R.I from 1.3 - 1.8. Example: Moonstone can exhibit a Vitreous luster.
Subvitrious Subvitrious luster is somewhat less glass-like.
Resinous Resinous luster is that of a substance formed from resin. Example: Amber
Greasy Greasy luster is seen on Serpentine
Waxy Waxy luster describes the luster observed on Jadeite and Nephrite.
Dull Dull luster describes the luster of Ivory

Pearl Luster is defined by the sharpness and brightness of reflections as seen on the surface of a pearl. The Gemological Institute of America classifies pearl luster into five grades.

Pearl Luster
Excellent Reflections are uniformly bright and sharp.
Very Good Slightly less defined, fairly sharp, bright reflections.
Good Reflections are not clear but still fairly bright.
Fair Reflections are very fuzzy and weak.
Poor Reflections are non-existent or extremely weak.