Most geologists, including the author of this article, have not thought as deeply about luster as gemologists. If you open almost any mineralogy textbook to the pages that describe a mineral, the luster is usually given as one or two of the adjectives listed above. For example: submetallic to metallic.
The author completed the coursework for a Graduate Gemologist diploma from the Gemological Institute of America in 2018. While taking his courses, he realized that gemologists put more work into their assessment of luster. They also use luster in gem identification in more ways than geologists use it in mineral identification. A gemologist might report:
 a general luster for a mineral (gem) species a general luster for a mineral (gem) variety a fracture surface luster a cleavage surface luster a polished surface luster |
In corundum, basal parting planes can exhibit a pearly or submetallic luster. This differs from the vitreous to adamantine luster that might be observed on crystal and fracture faces. A pearly luster on parting planes can indicate that the material might display asterism if cut properly.
Gemologists pay attention to luster because, after color, luster is the most obvious property of an item that will be sold for tens, hundreds, thousands, or millions of dollars.
Here’s a problem: You are examining a cabochon (a dome-shaped gem) cut from a material that might be green quartz, chrysoprase (green chalcedony), or dyed quartzite. You know that under a microscope (or a hand lens), the edge where the flat bottom of the cabochon meets the domed top often has at least one tiny chip. You find a chip with a conchoidal shape. How would you be able to tell if the cabochon is cut from green quartz, chrysoprase, or quartzite?
The answer is in the luster of the chip’s surface. These three materials have distinctive fracture lusters. Green quartz will be vitreous, chrysoprase will be dull to waxy, and quartzite will be granular.
The problem above was simple. The material could have been one of a large number of gem materials beyond quartz, chalcedony, and quartzite. It might have been jadeite, nephrite, idocrase (vesuvianite), serpentine, amazonite, prasiolite, apatite, heliodor, malachite, tourmaline, diopside, fluorite, a green garnet, gaspeite, emerald, green beryl, kyanite, maw sit sit, moldavite, opal, peridot, aventurine, sphene, spodumene, epidote, variscite, zoisite, or another less-common gem. One look at the luster might eliminate most of the green gems in this list.
Gemologists are also concerned about phenomena. These are things that gem materials do to light beyond a simple luster, such as: adularescence, aventurescence, iridescence, labradorescence, opalescence, play-of-color, and fire. If these are not related to luster, they can be hard to separate from it.
We will conclude with a comment about the luster known as “pearly”. There are many kinds of pearls, produced by different types of organisms, who live in different parts of the world, in different types of water. Gemologists who specialize in pearls can teach entire courses on the pearly luster.