Phenomena A few gemstone varieties exhibit special optical effects in visible light. We can identify gemstone from the effect. The characteristics are described in this follows :
- Adularescence (sheen) such as moonstone, this optical phenomena give the stone milky sheen or billowy or floating light effect. It is produced by light entering some transparent to traslucent stones in scattered internally by tiny particles or structural irregularities.
- Interference and Diffraction Colors ( Play of Color ) the interference of light reflected from the surface or from the interior of mineral may produce different colors as the angle of incident light changes. This internal iridescence produces the color effects in opal, and several gems.
- Color Change the best known example is alexandrite. It is sometimes referred to as the alexandrite effect. It shows a difference in color when examined under different lighting conditions.
- Chatoyancy (cat’s eye effect) a silky sheen that results from closely packed parallel fibers or needlelike inclusion, It’s surface displays aband of light. As the stone is tured, the narrow beam moves from side to side as does a cat’s eye.
- Asterism (star effect) like the cause of cat’s eye effect, if there are three identical chrystallographic directions, as in hexagonal crystals, silk (needlelike inclusion) may present parallel to each of the like directions. A cabochon cut from a crystal shows a beam of light perpendicular to each parallel set of inclusions. This triple chatoyancy is called asterism and is best known in star rubies and star sapphires.
- Aventurescence some gems have as inclusions relatively coarse to very fine disclike or platy crystals of another mineral. When such materials are illuminated by overhead light source, these inclusions may produce individual reflections, giving the stone an overall glitery effect.
Transparency (Light Transmission) Most of gemstones are light colored and usually transmit some light. The following terms are used to express the degree to which light is transmitted :
- Transparent: If the antline of an object seen though the stone is perfectly distinct.
- Semitransparent: If an object is seen but with indistinct outline.
- Translucent: If light is transmitted but objects cannot be distinguished.
- Semitranslucent: If light is transmitted only on thin edges.
- Opaque: If no light is transmitted even on thin edges.
Luster The luster of gems refers to the quantity and quality of light reflected from its surface luster as described by the following terms :
- Adamantine: the brilliant luster like that of a diamond.
- Subadamantine: between adamantine and vitreous.
- Vitreous: the luster of glass.
- Subvitreous: between vitreous and greasy.
- Greasy: oily appearance.
- Waxy: the luster seen on a candle or an unpolished fingernail.
- Dull: the luster common to unpolished.
- Resinous: the luster of resin (like amber).
- Silky: a silk like sheen caused by reflection of light from a patterned structure.
- Pearly: a sheen shown by pearl.