What Is Gemstone Luster?


gemstone luster - talc various
Talc’s luster can range from pearly to greasy to dull. “Talc,” Argonaut Quarry, Ludlow, Windsor County, Vermont 2, by Stephanie Clifford. Licensed under CC By 2.0.

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.

Adamantine

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

gemstone luster - adamantine
“Diamond,” 1.63 cts, 7.52mm, round brilliant cut. © Dan Stair Custom Gemstones. 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.

gemstone luster - vitreous
“Precious Topaz,” 1.11 cts, fancy brilliant oval cut, Topaz Mountain, USA. © Dan Stair Custom Gemstones. Used with permission.

Pearly

Gems with this type of luster show an iridescent play of colors on their surfaces, like the orient of pearls. Gypsum and charoite may show pearly luster.

gemstone luster - pearly
“Charoite (charoite-dominated potassic metasomatite),” 5.7 cm across, from the Cretaceous of Siberia, by James St. John. Licensed under CC By 2.0.

Silky

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

gemstone luster - silky
Ulexite is also called the “TV Stone” for its transparency. “Fibrous Ulexite” 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.

gemstone luster - greasy
“Graphite” 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.

gemstone luster - resinous
“Insects in Baltic Amber” 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.

gemstone luster - waxy
“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.

gemstone luster - dull
Rhodonite may have a dull luster, but its other qualities are amazing. “Mystic Journey” 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.

gemstone luster - metallic
“Gemstone Spiral Bracelets – Hematite” by Audrey B. 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.