Phenomenal gemstones are those which exhibit special optical characteristics, beyond their usual beauties. There are nine categories of phenomena in gemmology: asterism, chatoyancy, adularescence, aventurescence, labradorescence, play-of-color, iridescence, orient, and color-change. Unfortunately, few people are aware of the wonders of these seemingly magical gems.
Asterism is more commonly known as the ‘star’ phenomenon, with the most prized examples occurring in sapphires and rubies. It can, however, also appear in spinel, diopside, beryl, and quartz. This phenomenon is caused by tiny needle like inclusions which reflect light onto the surface of the stone. Stones with these types of inclusions are cut ‘en cabochon’ to display this optical quality. They can exhibit from four to as many as twelve rays, which radiate outwards from a central point.
Similar to asterism is another type of gemological phenomenon—chatoyancy. More commonly known as ‘cat’s eye’ gems, stones of this type exhibit one line of reflected light on their surface when cabochon cut, rather than multiple, intersecting rays. Chrysoberyl is perhaps the gem which is most typically associated with this phenomenon, however, like asterism, many varieties of gemstone can exhibited this effect, including apatite, tourmaline, beryl, and tiger’s eye quartz. Again like the star gems, the effect is caused by small tubular inclusions.
Adularescence is a phenomenon also caused by inclusions, however no rays are formed. In this case the inclusions reflect light which creates a hazy glow just below the surface of the stone. Perhaps the most recognized adularescent gem is the moonstone, celebrated for its bluish white milky radiance. Two other related gemstone phenomena are aventurescence, in which inclusions cause light to reflect in a glittery effect, and labradorescence, named for the gemstone labradorite, in which a broad flash of color is produced.
Play-of-color is a phenomenon that is only found in opals, which is, according to the Gemological Institute of America, the world’s most popular phenomenal gem. Play-of-color refers to the flashes of color exhibited by opals, which break up light into distinct spectral colors, caused by the grid-like structure of the stone. This can occur in many of the different varieties of opals—black, white, and fire opal, each named for the base color of the stone—however, opals can also occur without play-of-color. Some of the rarest and therefore more valuable opals are those which display all the colors of the rainbow, most notably red, against a black base color.
Iridescence is the term for an object’s ability to change color as it moves. Mother-of-pearl is one such substance which exhibits iridescence, as do iris and fire agates. Like opals, pearls also have their own phenomenon, a variant of iridescence, which is called orient. These organic gems typically have a base color—most typically white or cream—as well as an over tone—such as pink, blue, purple, or green. Orient is the name given a pearl with an iridescent or multicolored overtone.
One of the most amazing gemological phenomena, colour-change refers to gems which change color in different types of light. The most famous of these is alexandrite chrysoberyl, which changes from reddish purple in incandescent light to bluish green in fluorescent light. Other gems which can have the ability to change color are sapphire, garnet, and spinel.
Exceptionally rare, some stones exhibit multiple phenomena. Rainbow moonstone, for example, displays both the sheen of a moonstone and a multicolored flash. A number of star and cat’s eye gems—such as sapphire or alexandrite—can at the same time be colour-change. Whatever the effect or combinations thereof, phenomenal gemstones lend the jewels in which they are set an allure beyond the brilliance they already possess.
