| Optical Properties of Gems |
 |
| Luminescence: This incorporates a gemstone’s ability to emit visible light in darkness when exposed to ultraviolet light (fluorescence, named after flourite, the predominant flourescent gemstone), and in the case of kunzite, to produce an "afterglow" which lingers after the light has ceased (phosphorescence). Luster: It is caused by light reflecting from the stone's surface. The smoother and more highly polished the surface is, the greater the luster will be. High light refractivity of a gem will cause greater luster as well. The most intensive luster is seen in the highest refractive indices, diamond, zircon, and rutile, and is known as an adamantine luster. Hematite produces a metallic luster. Most gemstones have a vitreous or glassy luster, but there are other types of lusters, including resinous (amber), greasy (serpentine), waxy (turquoise), pearly (rhodonite), and silky (tiger's eye). Refraction:Lignt bends when it travels through a gemstone. When light hitting a gemstone splits into two rays traveling through the stone at different speeds and in different directions, the reaction is called birefringence or double refraction. This is seen uncommonly and in a variety of calcite called Iceland Spar as well as zircon, rutile, and sphene. Dispersion: is the separation of light into its separate spectral colors. Gemstones with the highest light refraction typically show the highest dispersion rate as well (rutile, sphene, diamond, zircon). This color dispersion or fire can be enhanced by an appropriate facetting style. Pleochroism: Color changes which are evident when viewed from different angles in gemstones (iolite, alexandrite, andalusite) is called pleochroism. Stones like ruby, sapphire, iolite etc. have two color shades when viewed from different angles. |