|Brilliance, Luster & Fire
You'll often hear the words brilliance, luster and fire used in describing fine gemstones. These are not just terms of approval used by those who know -- or think they know -- gemstones. Rather these are actually technical terms that have fairly precise meanings, even if they are often misused.
All three of these terms have to do with the visual appearance of a gemstone as it reflects light. Brilliance refers to the light reflected from the interior of a gem. Thus only a transparent gemstone can be said to have brilliance. Luster, on the other hand, refers to the light reflected from a gem's surface. If someone describes an opaque star ruby as 'brilliant,' for example, they are misusing the term. They need to talk about the ruby's luster instead.
The brilliance of a gem -- the amount of light reflected back out the crown to the eye -- is a function of the gem material's refractive index as well its cut. Gems with high refractive indices -- such as diamond, sphene, zircon, garnet, sapphire and ruby -- are usually cut to maximize their considerable brilliance. That means that the pavilion facets must be cut at the correct angles so that light will be reflected back out the crown instead of passing through the bottom of the gem. In gemstones with much lower refractive indices -- such as quartz -- it often makes more sense to optimize the cut for color rather than brilliance, since a cut optimized for brilliance would result in a very deep pavilion that would be harder to set in jewelry.
Where brilliance is measured simply in terms of 'more or less', there are a set of technical terms used by gemologists to refer to different types of luster. A gem surface that is reflective like polished gold or silver has a luster that is called metallic; only a few gems such as hematite and pyrite exhibit this quality. A diamond-like luster is known as adamantine, a quality shared not only by diamond, but also by some other gemstones such as sphene, demantoid garnet and zircon. A vitreous or glass-like luster is the most common among transparent faceted gems, including sapphire, ruby, emerald and tourmaline. Other lusters include silky (e.g., malachite), resinous (e.g., amber) and greasy (e.g., jade).
Fire refers to the tendency of a gemstone to split light into its spectral colors. The gemological term for this is dispersion. Dispersion occurs when different wavelengths of light are refracted by a different amount by the refracting medium. Diamond is famed for this quality, but a number of other gemstones are notable for their fire, including demantoid garnet, sphene and zircon.