|Singly Refractive Gems
Cubic Crystal Spinel
One of the signature properties of a gemstone is its refractive index. When light passes from one medium to another, the light rays are refracted or bent at an angle. It's why a straw standing in a glass of water appears to be bent -- light travels more slowly through water than through air.
Each substance has its own refractive index, and this fact is quite useful for gemstone identification. The refractive index of a particular material is simply expressed as the speed of light in a vacuum divided by the speed of light through the particular material. In case of water, the refractive index (or RI) is 1.33. For diamond, the RI is in the range of 2.417 to 2.419.
Some gemstones are singly refractive: they have only one refractive index. Other gemstones -- in fact, most -- are doubly refractive: they have two different refractive indices. When a beam of light enters a doubly refractive gemstone, it is split into two beams, each travelling at a different speed and on a different path through the crystal. Birefringence is a measurement of the difference between the two refractive indices in gems that are doubly refractive, and it ranges from a low of .003 to a high of .287.
Relatively few gemstones are singly refractive. The most well known ones are diamond, garnet and spinel. Indeed many people think these are the only singly refractive gems. But in fact the list is quite a bit longer, and can be organized into several different categories:
Organic Natural Amber
Cubic solids are very symmetrical, and have three equal and perpendicular axes. For this reason they refract light in all directions at the same velocity. Cubic crystals include not only diamond, garnet and spinel, but also fluorite, boleite, lapis lazuli, sodalite, hackmanite and pyrite. Gold and silver are also cubic crystals.
All of the gems which are amorphous -- having no crystal structure -- are singly refractive. There are several subcategories of amorphous gems: opals, natural glass, and organic gems.
All the members of the opal family -- common opal, precious opal, fire opal and boulder opal -- are composed of hydrated silicon dioxide, a kind of hardened jelly. Opals have a significant water content, usually between 3 and 10%.
Natural Glass Obsidian
The natural glasses include obsidian and moldavite. They are amorphous because they formed under conditions of rapid cooling, which prevented crystal growth.
The organic gems include amber, jet, ivory, pearl and ammolite. Amber and jet are derived from plant materials, while ivory, pearl and ammolite have their source in the animal kingdom. Some gems which appear to have an animal source, such as fossilized coral or petrified wood, are actually crystalline, since the original organic material has been replaced by jasper or chalcedony.