May 27 2012
Phenomena are unusual optical effects that occur in some gem species. Phenomenal gems are often fashioned as cabochons or tablets to display their phenomena. To look for a phenomenon I would suggest that moving a strong concentrated light source, such as a penlight or fiber-optic light, above the stone and look for the following phenomena:
Adularescence (Ad)—A billowy light seen in moonstone varieties of feldspar. Caused by diffraction, interference, and diffusion of light by minute concentrations within the stone of another feldspar with a refractive index (RI) slightly different from the main mass.
Aventurescence (Av)—A colored spangle or glitter effect produced by light reflecting from platelets. Seen in goldstone glass, aventurine quartz, and sunstone feldspar.
Asterism effect in Diffusion star Sapphire
Asterism (A)—A star effect usually produced by light reflecting from minute, precisely oriented, needle-like inclusions. In a cabochon, asterism looks like several arms or rays of light radiating from a common center.
Chatoyancy (C)—Cat’s-eye effect caused by light reflecting from minute, parallel, needle-like inclusions, as seen in cat’s-eye chrysoberyl. Can also be caused by internal fibrous structure, as in tiger’s-eye quartz, or by parallel tubes, as in cat’s eye tourmaline.
Iridescence (I)—The presence of interference colors inside or on the surface of a gem. Caused by layers or gaps in the material that break white light into spectral colors. Seen in fire agate and iris agate, it can resemble an oil slick on water.
Color change (CC)—A distinct difference between a gem’s bodycolor in incandescent light and its color in daylight or fluorescent light. Seen most notably in alexandrite chrysoberyl.
Labradorescence (L)—A broad color flash seen across the surface of labradorite feldspar. Caused by light interference by a thinly layered twinning structure.
Orient (O)—A form of iridescence found on or just below the surface of a pearl, cultured pearl, or mother-of-pearl shell. Layers of minute crystals on the pearl surface separate white light into spectral colors.
Play of color in Ethiopian Opal
Play-of-color (P)—Flashes of spectral colors in opal, produced when layers of minute spheres and voids break up white light. Opal is the only gemstone that display phenomenon "play-of-color".