Color Color is the single most important factor when evaluating colored gems. Basically, the more attractive the color seen, the higher the value. Bright, rich and intense colors are valued over those that are too dark or light. Colors that are dulled by tones of black, gray or brown are regarded as less desirable. The colors seen should ideally remain attractive regardless of prevailing light conditions. Whether viewed indoors, outdoors, by day or by night, a gem should always remain beautiful. The mixing of color hues into combinations, such as purple-blue in Tanzanite and bright blue-green in Apatite is attractive and value enhancing. Although specific colors hues can affect the prices of gems, personal preferences are also very important. Many gems also have specific expressions to denote the very top colors found within a species. For example, Pigeon Blood Red or Cornflower Blue are respectively used for Ruby and Sapphire. Such terms are equable with the most desirable colors to be found within a gem species (i.e. colors that cannot be bettered).Optical EffectsSome gems exhibit unusual optical effects known as phenomena. These rare and beautiful effects very often add value to gems. The Cat’s Eye Effect, the Star Effect and the Color Change Effect are very popular phenomena and are highly valued.
Chatoyancy Chatoyancy or the Cat’s Eye Effect is a reflection effect that appears as a single bright band of light across the surface of a gemstone. This phenomenon is commonly found in Chrysoberyl, Tourmaline and Tigers Eye.
Asterism Asterism or the Star Effect is a reflection effect that appears as two or more intersecting bands of light across the surface of a gem. This phenomenon is commonly found in Ruby, Sapphire and Garnet.
Color Change Color change gems are those that distinctly change their color when viewed under two different light sources. This phenomenon is commonly found in Alexandrite, Sapphire and Color Change Garnet.
Iridescence Iridescence is the rainbow like color effects seen in some gems. This phenomenon is commonly found in Opal, Ammonite and Moonstone.
Aventurescence Aventurescent gems are those that have a colorful play of glittering reflections from small metallic inclusions. This phenomenon is commonly found in Sunstone, Aventurine Quartz and Gold Stone.
Unlike Diamonds, colored gems possess variable optical properties and are not cut to a uniform ideal. A well-cut colored gem exhibits even color, a minimal number of inclusions, good brilliance and shows the majority of Carat weight when viewed from the top. A well-cut gem should also exhibit good symmetry and polish condition. Facets should be aligned straight in relation to the gem’s girdle and also to each other. Polish condition should be good with no visible surface pits and polishing lines.
Generally, rare gems are more highly prized than more common varieties. However, if a gem variety is so rare that it is essentially unknown to the general public, it is often classified as a "collector gem". Gems such as Boracite, Childrenite and Simpsonite are extremely rare, attractive and durable, but they are unlikely to command prices appropriate to their rarity because fewer people are aware of their existence.