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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 gemstone jewelry.

Luster (external brilliance)

Luster is the quality and quantity of light reflected from a gem’s surface and is a function of refractive index and surface perfection (polish).

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 luster (a mirror). If much of the light is absorbed by the gem, resulting in a dull reflection, it’s luster is said to be low. Commonly used terms to describe luster include:

Metallic: This is the very high luster shown by metals such as gold and silver, and by gems such as hematite and pyrite.

Adamantine: Very bright and reflective, almost metallic, luster as displayed by diamonds. The descriptive term sub-adamantine is also sometimes used to describe gems with a bright luster closer to diamond (e.g., demantoid garnet).

Vitreous: The luster 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 luster.

Silky: Some fibrous minerals such as gypsum and malachite have a silky luster.

Pearly: Pearls are composed of layers of nacre from which light is reflected at and near the surface.

A common misconception is that high luster is important. Certainly for colorless gems such as diamond, this is the case, but for gems that depend on their color for their beauty, a high luster is actually an impediment, since it prevents light from entering the gem. This is why the colors of high refractive index (RI) gems such as diamond are often somewhat “steely,” while the colors of low RI gems such as emerald can be so beautiful.

Brilliance

Brilliance refers to the amount of light returned to the eye from the interior of a gem and is mainly a function of refractive index, proportions and transparency.

Faceted gems are designed to catch all possible light and throw it back to the eye. If the gem is cut too shallow, light from above and below passes straight through, creating a window where one can see through the gem. Thus brilliance suffers.

Similarly, if the gem is cut too deep, light from above passes out the side, creating areas of darkness, known as extinction. Again, brilliance suffers (and there’s nothing worse than suffering brilliance).

A gem’s potential brilliance is mainly a function of refractive index (RI), with higher RI gems having more potential brilliance. But as we learned with luster, a high RI can actually hurt the display of color. Thus one should not worry too much about RI. The main thing is to look for stones that are of good proportions, not too shallow, nor too deep. Overly deep stones have the double disadvantage of being more difficult to set in jewelry.

It is important to note that gem cutting is an art form rather than a formula. What works best is simply what works best—that which brings out the greatest beauty in the gem. Since judgment of beauty is by definition subjective, quality of cutting is also a matter of opinion.

Fire

Fire (or dispersion), is the splitting of light into its component colors (i.e., red, orange, yellow, green, blue and violet light). As light passes from one medium to another its individual colors are bent by different amounts. The resulting effect is that the light no longer appears white, but is broken into separate colors. Sphene, demantoid garnet and diamond all show strong dispersion. Dispersion is also a function of stone size, with larger gems being able to spread light more because of the longer light paths.

luster-us-zoultier
Luster is the surface reflection of light

well-cut-us-zoultier
A well-cut gem should maximize the light reflected through the top of the gem

too-shallow-zoultier
Too shallow a cut and light passes straight through the gem

too-deep-zoultier
Too deep a cut and the light will not be returned to the eye, instead exiting the side

fire-us-zoultier
"Fire" is the splitting of light into its component colors