Lustre (also spelt luster) refers to the surface reflection of light off an object and is a description often used in gemmology to describe the way light interacts with the surface of the gem. Black Diamonds for example, don’t display any dispersion or brilliance, but demonstrate as much lustre as a white Diamond.
In fact Black Diamonds and Marcasite often out sparkle and out dazzle colourless Diamonds because their lustre can be greater than the internal brilliance and dispersion of white Diamonds.
Opaque gems are normally cabochon cut to maximise their lustre, while gems that are transparent to translucent are often step cut or brilliant cut in order to take advantage of their internal brilliance and their ability to disperse light. That said, step cut and brilliant cut gems still retain the ability to show lustre off their crown and table facets.
The harder the gemstone and the greater its density, the better the lustre tends to be. Diamonds and Zircons, for example, have a gorgeous, sparkling lustre known to gemmologists as an adamantine lustre.
The most common lustre in faceted transparent gemstones, is similar to that seen in a pane of glass and is known as a “vitreous lustre”.Aquamarine, Spinel, Topaz, Emerald and Tourmalines are amongst those that are said to have a vitreous lustre.
Gems with a greasy lustre are those whose surface reflection is similar to that of grease. This is normally caused by a mass of microscopic inclusions within the mineral. Peridot, Alexandrite, Opals and some Garnets are said to have a greasy lustre.
Metallic lustre is similar to that of a polished metal or that of a mirror; Pyrite and Marcasite are classic examples.
Gemstones with a pearly lustre obviously have an appearance similar to that of an organic Pearl. Their appearance is normally due to layers within the gem, from which light reflects in an unorthodox, yet beautiful manner. Opals have a pearly lustre, which is often referred to as Opalescence.
Resinous lustre is similar to the appearance of resin or chewing gum. Amber is one of the most well-known gems that is said to have a resinous lustre. Others include Titanite and Vesuvianite.
Gems with a silky lustre have very fine fibres (just like silk) which are arranged parallel to each other. Malachite and Sillimanite are both said to have silky lustre. A fibrous lustre is similar to a silky lustre, but has a coarser texture; Tiger’s Eye has a fibrous lustre.
As the name suggests, gems with a waxy lustre have an appearance similar to that of wax. Jade and Turquoise have a waxy lustre.
To really achieve the most enjoyment possible out of your gem collection, it helps if you can truly understand the difference between lustre, brilliance and fire. Try reading up on all three of these topics and then take out your gem collection and try and identify all three of these different visual effects. Once you can do this, you will find that your appreciation of your collection will grow immensely.
