Chatoyancy
is an effect resembling a slit of cat's eye. Chatoyancy is a result of tiny fibers, or needles, naturally arranged in a parallel configuration.If the gem is cut en cabochon so that the base is parallel to the fibers, when rotated in a single light source, the "eye" slides over the surface of the gem. Officially, the term cat's eye used alone refers to crysoberyl.
Asterism
is the phenomena commonly known as "star". Star sapphire may be the most common. The rays of the star form very definite angles, depending on the crystal, as a result of rutile, or other tiny needles are aligned at angles.Gems are usually cut en cabochon to show asterism. Six rayed, four rayed and 12 rayed stars are possible.
Adularescence
Moonstone is a prime example of this phenomena which is caused by interference cause by the mineral's lamellar structure.The result is a blue-white opalescence which moves over the surface as a cabochon is turned in light.
Aventurization
Nature's metallic glitter is apparent in minerals such as Aventurine Feldspar. Small leaf-like inclusions of hematite, fuchsite or goethite in opaque minerals create the colorful reflections.Iridescence is a play of color in gems caused by dispersion of light. The effect is a result of flaws, strains or cracks. The effect can be produced by producing cracks in rocks.
Labradorescence
is a metallic appearing play of color resulting from interference or twinned layering.Opalescence
A "pearly" appearance, usually "milky blue" caused by reflection of short-wavelength light .. mostly in the blue spectrum. Not to be confused with opalization.Opalization
Play of color in opal which changes depending on the angle viewed when the stone is rotated in light.The play of color is interference of light created by clusters of microscopic cristobalite spheres (.0001 mm) included in silica gel.
Silk
Fine fibrous inclusion or channels reflecting light cause a silk-like appearance. The effect is not desired in faced stones as it reduces transparency.Opaque stones stones properly cut can show a cat's eye.
Luminescence
is a catch-all description to describe the emission of visible light on exposure of certain other rays; or chemical and physical reactions .. but does not include pure heat radiation.One example is the luminescence of some gems when exposed to ultraviolet light, and called fluorescence.
If the specimen continues to emit light after the effect has been removed, the result is said to be phosphorescence.
Fluorescence
in gems is thought to be caused by small metal impurities such as chromium, manganese, cobalt, nickel, molybdate, tungsten and certain uranium compounds.Iron, when present, prevents fluorescence.