In certain gemstones, notably opals, brilliant areas of color can be seen within the stone; these areas change in hue and size as the stone is moved. This phenomenon, known as play of color, differs from fire and is caused by interference and reflection of the light by tiny irregularities and cracks inside the stone. Opals also exhibit milky or smoky reflections from within the gem. Gems that are fibrous in structure show irregular interior reflections similar to those seen on watered or moiré silk. This optical property, which is called chatoyancy, is exhibited by several gems, notably the tigereye and cat’s-eye.

The appearance of a gem as seen by reflected light is another optical property of gemstones and is called luster. The luster of gems is characterized by the terms metallic, adamantine (like the luster of the diamond), vitreous (like the luster of glass), resinous, greasy, silky, pearly, or dull. Luster is particularly important in the identification of gemstones in their uncut state.

A gem cannot always be identified by sight alone. It is therefore necessary to rely on measurement of the optical properties that can be determined without harming the stone in any way.

The gemologist uses an instrument called a refractometer to measure the characteristic property of the stone, known as refractive index, which is its relative ability to refract light. In addition, an instrument called the polariscope is employed to determine whether a gem is doubly or singly refracting (see Crystal). Emeralds, rubies, sapphires, amethysts, and synthetic rubies and sapphires are all doubly refracting, whereas diamonds, spinels, synthetic spinels, garnets, and glass are singly refracting. A special dark-field illuminator with a binocular microscope is employed for examining the interior of a gemstone to determine whether it is of natural or artificial origin, and to search for inclusions characteristic of a given gemstone.

These tests usually are sufficient to identify the rather limited number of materials used as gemstones; occasionally, however, other instruments are required, including a dichroscope, which measures the property called dichroism, or a spectroscope to determine the characteristic absorption spectra (see Spectroscopy; Spectrum). Hardness, the test ordinarily associated with gem testing, is never used on cut stones by the gemologist.

Another physical test that can be given to an unknown stone is the determination of its specific gravity (see Density). For exact determinations various weighing devices are used, but rough approximations of the specific gravity of lighter stones can be made by means of a series of liquids of known specific gravity. If the stone will float in a liquid having a specific gravity of 4 and sink in a liquid with a specific gravity of 3, the specific gravity of the stone must lie between these limits and be approximately 3.5

Amethyst
Aquamarine
Beryl
Birthstone
Bloodstone
Carnelian
Cat’s-Eye
Chalcedony
Chrysoberyl
Emerald
Garnet
Heliotrope (mineral)
Jade
Jasper
Onyx
Opal
Pearl (gem)
Olivine
Ruby
Spinel
Rutile 
Sapphire
Sardonyx
Tourmaline
Turquoise
Zircon