Inclusions and optical phenomena
Much of the beauty of gemstones is due to the way they react when light shines on them. Some gemstones exhibit special optical effects that adds to their value; others may have inclusions that increase their appeal and rarity.
The surface appearance of a gemstone is referred to as its lustre. Most transparent gemstones have a vitreous or glasslike lustre. Other terms used to describe lustre include adamantine, or diamond like (diamond and demantoid garnet), metallic (hematite), waxy (turquoise), greasy (topaz), resinous (amber), silky and dull. Polishing, waxing and oiling can enhance the lustre of a gemstone.
Gemstones with oriented inclusions such as fibrous or needleike crystals or cavities may be cut as a cabochon, with a polished domedtop surface, to reveal a cat’s-eye (with a single bright line) or star. Cart- eye gemstones are also called chatoyant, after the French word for cat (chat). The cat’s-eye is a result of light reflecting off internal structures. such as a single set of parallel inclusions. Cat’s-eye gemstones include quartz, chrysobery and garnet.
Interference patterns, such as you might see on the surface of an oil spill on the road, are caused in gemstones by reflection off internal surfaces. Light is split into the colours of the rainbow, some or all of which can be seen on the surface of the gemstone without the need to cut it as a cabochon, though the features may be enhanced by polishing. The play of colour of precious opal, also called iridescence, is a result of light reflecting off patterns of spheres within the gemstone. The iridescence seen in labradorite, due to light reflecting off thin internal layers, is called adularescence, schiller or sheen. Other gemstones that show iridescence include hematite, iris quartz and quartz.
As a result of the reflection of light off more than one set of parallel inclusions, a gemstone cut as a cabochon may reveal a star stone (asterism). Star stones with six rays (three sets of parallel features) are the most common, but they may have four (two sets of parallel features) or 12 or more rays. Star stones include quartz, corundum and garnet.