II.

Optical Properties

The beauty of gems depends to a large extent on their optical properties. The most important optical properties are the degree of refraction (see Optics) and color. Other properties include fire, the display of prismatic colors; dichroism, the ability of some gemstones to present two different colors when viewed in different directions; and transparency. Diamond is highly prized because of its fire and brilliancy, ruby and emerald because of the intensity and beauty of their colors, and star sapphire and star ruby because of the star effect, known as asterism, as well as for their color.

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.