Gemology
Light & Color Effects in Gemstones
A number of gemstones show unique light effects which are not related to their color. Neither are the effects cause by the gemstone's normal chemical makeup. The effects are caused by factors such as refraction, reflection and interference.
Chatoyancy
Chatoyancy is an effect resembling the slit of cat's eye. The phenomena 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" when used alone refers to crysoberyl.
Asterism
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. Asterism results when 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
A Moonstone is a prime example of this phenomena which
is caused by light interference. That light interference is caused by the mineral's lamellar structure.
The result is a blue-white opalescence which moves over the surface as a cabochon when the material is manipulated in the presence of 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
Labradorescence is a metallic appearing play of color resulting from interference or twinned layering.
Opalescence
Opalescence results as 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
Opalization is a 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 faceted stones as it reduces transparency.
Properly cut opaque stones with silky fibers cut can show a cat's eye.
Luminescence
Luminescence is a catch-all description for 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
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.