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.
gemstone luster - talc

Talc can show a range of luster, from pearly to greasy to dull. Photo by Stephanie Clifford. Licensed under CC By 2.0.

Table of Contents:

  • Types of Gemstone Luster
    • Adamantine
    • Vitreous
    • Pearly
    • Silky
    • Greasy
    • Resinous
    • Waxy
    • Dull
    • Metallic
  • The Concept of Luster in Gemology
  • What is Polish Luster and Fracture Luster?

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.

Adamantine

This term describes gems with a brilliant, mirror-like appearance, like diamonds.

diamond - adamantine luster

Diamond. Photo © Joel E. Arem, PhD, FGA. Used with permission.

Vitreous

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.

topaz - vitreous luster

Blue topaz. Photo © Joel E. Arem, PhD, FGA. Used with permission.

Pearly

Gems with this type of luster have surfaces that look like that of pearls. Some may even show iridescent colors on their surfaces, like the orient of pearls. Gypsum and charoite may show pearly luster.

gypsum - pearly luster

Gypsum crystals in a “desert rose” cluster. Photo courtesy of liveauctioneers.com and Michaan’s Auctions.

Silky

Some gems, like ulexite, show fine parallel threads that look like the texture of fabric. This is known as a silky luster.

ulexite - silky luster

Ulexite is sometimes called the “TV Stone.” If polished perpendicular to its fibers, a ulexite can project an image of what’s behind it onto its front. Photo by John Bell. Licensed under CC By-SA 2.0. (Cropped from original).

Greasy

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.

graphite - greasy luster

Graphite. Photo by Ryan Somma. Licensed under CC By-SA 2.0.

Resinous

Amber consists literally of preserved prehistoric plant resin. Therefore, it should come as no surprise that this gem material has a resinous luster. Sphalerite gems may have a resinous luster, too.

amber - resinous luster

Amber may have inclusions of prehistoric creatures, like the fly and mosquito in these pieces. Photo by Brocken Inaglory. Licensed under CC By-SA 3.0.

Waxy

Gems that may show this luster, like turquoise and opal, appear to have a layer of wax on their surface.

opal - waxy luster

Opal, 1.47 cts, Barion emerald cut, Lambina Mines, Australia. © Dan Stair Custom Gemstones. Used with permission.

Dull

A dull luster simply means a gemstone reflects little light, such as rhodonite or kaolinite.

rhodonite - dull luster

Rhodonite may have a dull luster, but its other qualities are amazing. Photo by Laura. Licensed under CC By 2.0.

Metallic

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.

hematite - metallic luster

Hematite bracelet. Jewelry and photo by Audrey. Licensed under CC By 2.0.

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.