Most of us gem lovers are especially intrigued by the unique optical properties of various gemstones (like the flashy blue schiller in these Labradorite Nuggets). In this post I will define and give examples of the some of the most fascinating optical properties of gemstones.


Refractive Index is a measure of how much the light refracts (breaks up) when entering the stone. "From Gemstones of the World" by Walter Schumann : "Most of us, when children, noticed that when a stick was immersed in water at a slant, it appeared to 'break' at water level...what we observed was caused by the refraction of light." Generally in transparent stones the higher the refractive index, the greater the luster and brilliance. Diamond, Zircon, and Sapphire all have relatively high refractive indexes.

As can be seen in this image of Blue Zircon Faceted Round Beads the stones have great luster and brilliance, due to the high Refractive Index as well as Dispersion. Color dispersion describes the process of white light separating into it's spectral colors. It is what causes the "fire", flashes of color, observed in diamonds. Diamonds, Cubic Zirconia ( or "CZ" a man-made stone) and Zircon (a natural stone), are examples of gems with high dispersion.


Pleochrosim describes how some stones will display different colors or depth of color when viewed from different angles. The stone may appear to change colors when moved around in the light. When cutting stones with strong pleochroism a cutter will need to carefully consider this and find the angles that display the best color. While often pleochroism can be problematic, sometimes the pleochroism is very desirable, and in that case the cutter will try to empasize the color dance effect. Gems with strong pleochrosim include Iolite, Andalusite, and Oregon Sunstone. The above photo shows Andalusite Faceted Heart Briolettes with an earthy mix of rosy browns and mossy greens.


Silk is an effect caused by the reflection of light by fibrous or needle-like inclusions. A similar phenomena is known as Chatoyancy where the silky appearance causes a cat's eye effect. The Chatoyant Scapolite Faceted Coins in the above image is an illustration of intense silk and cat's eye effect. Silk is especially desirable in Ruby and Sapphire. Crysoberyl, Tiger Eye, and Aquamarine are some of the many stones which will sometimes display chatoyancy.




Adularescence is a term used to describe the satiny sheen or "billowy light" that glides over the surface of Moonstone species. Note the satiny sheen on these Grey Moonstone Faceted Leaf Cut Briolettes----or the luminous gleam on these Ceylon Moonstone Heart shaped briolettes.

Schiller is the term for metallic iridescence or "flash" we covet so much in the Labradorite species. Most familiar is the blue-green shimmer against a grey base for example is these Labradorite Faceted Pear Briolettes