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Luster

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 relatively 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.

Luster

LusterDescription
MetallicMetallic 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.
SubmetallicSubmetallic luster is slightly less lustrous than metallic.
AdamantineAdamantine luster is the luster exhibited by Diamonds and gems with a Refractive Index of 1.9 – 2.5.
SubdamantineSubadamantine luster is slightly less than that of a diamond. Example: Zircon.
VitreousVitreous is defined as a glass-like luster. Gems with a R.I from 1.3 – 1.8. Example: Moonstone can exhibit a Vitreous luster.
SubvitreousSubvitrious luster is somewhat less glass-like.
ResinousResinous luster is that of a substance formed from resin. Example: Amber.
GreasyGreasy luster is seen on Serpentine.
WaxyWaxy luster describes the luster observed on Jadeite and Nephrite.
DullDull 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

GradeDescription
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.