Adamantine. Silky. Greasy? Gemologists use these and other evocative terms to describe gemstone luster. This simply means how a gem’s surface looks when it reflects light. For example, the brilliant mirror-like appearance of a diamond is called adamantine.
Types of Gemstone Luster
In the mineral world, luster comes in two main types: non-metallic and metallic. In addition, an intermediate type, sub-metallic, is sometimes used as a description. However, the gem industry most commonly deals with the non-metallic varieties. Most gems don’t meet the criteria for metallic or sub-metallic luster.
This term describes gems with a brilliant, mirror-like appearance, like diamonds.
Most gemstones, including popular jewelry species like quartz, topaz, and tourmaline, have a shiny, “glass-like” luster. In some reference works, you may encounter “glassy” as another term for this type of luster.
Some gems, like ulexite, show fine parallel threads that look like the texture of fabric. This is known as a silky luster.
Gems with a greasy luster seem to have a layer of oil or fat on their surface. Examples of this luster include graphite and green serpentine.
A dull luster simply means a gemstone reflects little light, such as rhodonite or kaolinite.
Metallic luster, a reflective metal-like appearance, is a term not usually used for gemstones. Hematite, however, is a notable exception. It has a striking, metallic sheen, and gem cutters have carved cameos and made beads from this material.
The Concept of Luster in Gemology
Although luster is a basic descriptive parameter for minerals, it can vary even within a single crystal. Due to the state of aggregation of the mineral, you may see differences depending on which crystal face you examine. For example, gypsum may have vitreous luster on some crystal faces but pearly luster on surfaces parallel to the cleavage. Furthermore, if the gypsum occurred in aggregates of long fibers, it would show a silky luster. Thus, luster may not make a useful diagnostic property for identifying gypsum or other gems!
You might find the lusters of some gem species described as ranges. For example, serpentines may have resinous, pearly, or waxy lusters. Sphalerites can range from resinous to even adamantine. In addition, some gemstone lusters have “sub-types.” These terms describe gems that come close to the main luster type. For example, chromite is a sub-metallic gem, while andalusite is sub-vitreous. Sub-adamantine gems include stolzite, monazite, and vanadinite.
Describing gemstone luster involves some subjectivity. This further limits its use for gem identification.
What is Polish Luster and Fracture Luster?
Gemstone luster generally refers to a gem’s base appearance. However, the gem’s condition may affect its luster.
A gemstone can have a polish luster (its appearance when polished) that varies greatly from its base luster. For example, polishing can transform jet, with a dull or waxy base luster, to vitreous.
A gemstone’s fracture luster describes how its fractures look when they reflect light. In some species, this may vary somewhat from its base luster. For example, vitreous spinel may have sub-adamantine fracture luster.