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
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.