We talk about brilliance, color and clarity all the time when we describe gems. These are the results of various optical properties of gems. Let's discuss some of these properties and how they effect our perception of a gem.  These properties often help us identify the gem type and their value.  We'll also try to clear up some common misconceptions.

Refractive Index: The property of a mineral that measures the difference in angle between external and internal light.  This is often linked directly to the brilliance of a gem, but is only indirectly related to brilliance. Gems with high refractive indices are more likely to be brilliant, but the accuracy of the cutting is the most direct factor to brilliance.

Double Refraction: The property of a mineral where a light wave splits when passing through the gem. This is often linked with brilliance as well.  It can be a distinct factor, but in extreme cases double refraction can be blurry or disorienting to the viewer.  Good cutters take advantage of double refraction and orient the gem to downplay the ill effects.

Dispersion: The characteristic of a mineral where the light is fanned out to display a rainbow of colors.  This is a difficult property to measure but is quite noticeable with gems that have a high rating.  Diamonds are famous for this effect (aided by cutting techniques), but there are a handful of gems that blow Diamonds away.

Pleochroism: This is the characteristic of a mineral that shows different colors depending on orientation.  Most often we will refer to dichroism which is a subset of this characteristic where 2 different colors are visible when viewed with different orientation.  Dichroism is confused with "color change" but in reality they are quite different.

Color Change: The characteristic of a mineral that shows different colors depending on the type of light.  This characteristic is quite rare and amazing in the minerals that have it.  Remember that for a "color change" gem the color will change regardless of orientation.  View a gem in fluorescent lighting, then shine a flashlight at it. If the color changes significantly, you've got one.

Opalization: or “color play”. The display of color like that found in precious Opals. There are many different names for this: "pinfire", "harlequin", "mosaic", "jelly". Don't call this "opalescence".

Opalescence: the creamy or silky luster of translucent gems.  This is usually caused by large amounts of very small inclusions giving the gem a translucent "glow".

Asterism: the optical phenomenon where gems cut in cabochon show rays off a central point called "stars".

Chatoyancy: the optical phenomenon where gems cut in cabochon show a single central ray that moves relative to the light source.  Called "Cat's eye".