Color plays the crucial role in determining the quality of gemstones. This feature is the most obvious. However there are also other optical properties of gemstones which are dependent upon light.

To the optical features of gemstones belong:
* Luminescence
* Luster
* Refraction
* Dispersion
* Pleochroism
* Refractive Index

Luminescence
Luminiscence is the ability of gemstone to emit visible light in darkness when exposed to ultraviolet light.

Fluorescence was named after flourite which is the predominant flourescent gemstone.

Phosphorescence is the ability to produce “afterglow” which lingers after the light has ceased. The example of such gemstone is kunzite.

Luster
The luster or brilliance is the main feature of transparent gemstones. It is caused by the reflection of the light from the surface of the stone. The smoother and more highly polished the surface is, the greater the luster will the gemstone have.

Due to high light refractivity the gemstone will have a greater luster. The most intensive luster which is called an adamantine luster is produced by such gemstones as diamond, zircon and rutile. A metallic luster can be produced by hematite which is not transparent.

Some types of gemstones may have a vitreous or glassy luster. The other types of luster are resinous (amber), greasy (serpentine), waxy (turquoise), pearly (rhodonite) and silky (tiger's eye).

Refraction
By the refraction you should understand the bouncing around of light from the greater part of the light ray which hits the gemstone and passes into the stone. When it appears in the denser medium of the gem the light bends. The amount of bending produces a measurable index which is called refractive index. This index is used to identify the gemstone.

The birefringence or double reaction is created when light hitting a gemstone splits into two rays. They move through the stone at different speeds and in various directions. The examples of such gemstone are Iceland Spar, zircon, rutile and sphene.

Dispersion
Dispersion appears due to the separation of light into its separate spectral colors. The highest dispersion can be displayed by the gemstones with the highest light refraction such as rutile, sphene, diamond, zircon. In order to create color dispersion or fire, a gem cutter should use a special facetting style.

Pleochroism
Pleochroism is color changes which can be seen if you view gemstone from different angles. Iolite, alexandrite, andalusite have this feature. Two color shades have such gems as ruby and sapphire which are also pleochroic. For example ruby has yellow-red and purplish-red, which distinguishes it from garnet and red spinel. Iolite may show lavender-blue, gray and pale yellow.