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Gems TV

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LUSTRE BRILLIANCE

These frequently used terms to describe the visual appearance of the interaction between light and a gem are often confused and misused, sometimes even by experienced industry professionals! The following clears up the confusion, allowing you to understand exactly what you are seeing when you gaze upon your gemstones.


Lustre

Lustre is the external reflection effect and is the amount of light that is reflected from a gem’s surface.

If nearly all of the light that falls upon a gem is reflected, resulting in a very bright reflection, it is said to have a “high lustre” (a mirror). If much of the light is absorbed by the gem, resulting in a dull reflection, it is said to have a “low lustre” (a brick). Commonly used terms to describe lustre include:

  • Metallic: This is the very high lustre shown by metals such as gold and silver, and by gems such as Hematite and Pyrite.
  • Adamantine: Very bright and reflective, almost metallic, lustre as displayed by Diamonds. The descriptive term “sub-adamantine” is also sometimes used to describe the lustre of gems with a bright lustre closer to Diamond (e.g. Alexandrite and Demantoid Garnet).
  • Vitreous: The lustre seen in polished glass and in most transparent gemstones whose refractive indices fall within the middle range of values (e.g. Emerald and Tourmaline).
  • Resinous: Certain gems that have low refractive indices, like Amber, have a resinous lustre.
  • Silky: Some fibrous minerals such as Gypsum and Malachite have a silky lustre.
  • Pearly: Pearls are composed of layers of “nacre” from which light is reflected at and near the surface.


Brilliance

Brilliance is the internal reflection effect and is the amount of light that is reflected from a gem’s interior. Brilliance is dependent on a gem’s optical properties (i.e. refractive index) and cutting (remember, coloured gems are not cut to a uniform ideal). Brilliance also depends on transparency (determined by a gem’s colour and optical properties), clarity, polish, lustre and wear.

Certain cutting angles allow the light to pass straight through the gem (commonly called a “window”) or to be deviated to the side. Accurate cutting to optimum angles will allow light to travel back towards the viewer when looking directly into the table facet.

Gems with low refractive indices (e.g. Quartz, Beryl, Iolite etc.) are not usually cut to show maximum brilliance as a very steep pavilion would be required. This would make the resulting gems too deep to be set into jewellery.

It is important to note that not all gems have the same optimal cut. Although it is true that many gems demonstrate optimal brilliance when cut to the same proportions as a perfectly round brilliant cut Diamond, other gem types benefit from either deeper or shallower pavilions. Optimizing the cut (faceting) for each gem type is an art in itself. Gems that are not cut correctly, either through a lack of experience or due to the gem cutter maximizing the carat weight instead of beauty, will result in a gem that doesn’t display its optimum brilliance.


Fire

Fire (or dispersion), is the splitting of light into its component colours (i.e. red, orange, yellow, green, blue and violet light). As light passes from one medium to another its individual colours are bent by different amounts. The resulting effect to the eye is that the light no longer appears to be white, but appears as separate colours. Sphene, Demantoid Garnet, Diamond, Zircon and all gemstones of a large size exhibit fire. It is a desirable property in gemstones, adding both beauty and value.




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