OPTICAL PROPERTIES OF GEMSTONES - gemandgemstone.com

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Color: Color of a gem depends on the way it absorbs light. White light is made up of seven colors and when it strikes a gemstone some of the colors are absorbed by the gemstone and the color or colors which are not absorbed and are reflected back gives the gem its color. When it comes to the color of the gemstone, there are two following types of gemstones:

Idiochromatic Gems:	They are “self-colored” gems, where their color comes from the elements which is an essential part of its chemical composition. Thus these gemstones show only one color or a narrow range of colors. For e.g. “Peridot” which is always green.
Allochromatic Gems:	They are “other-colored” gemstones, which are colored by trace elements or impurities that are not essentially a part of their chemical composition. For e.g. Ruby and different colors of sapphires. Allochromatic gemstones are often adjustable to color enhancements or change.

Parti-colored Gems: A gemstone that is made up of different-colored parts is known as parti-colored. These gemstones can be made of two colors (bi-colored), three colored (tri-colored) or even more. Different varieties of tourmaline show the best examples of parti-coloring, which can present as much as fifteen different colors or shades within a single crystal.

Pleochroic Gems :

Gemstones that show one color from one direction and other color or colors from other directions are pleochroic gemstones. Amorphous and cubic stones show only one color only; tetragonal, hexagonal and trigonal stones show two colors (dichroic); orthorhombic, monoclinic and triclinic stones can show three colors (trichroic).

Refractive Index (RI):

When a ray of light meets the polished surface of a gemstone, some of the rays are reflected and most of the rays passes into the gemstone. As gems have different optical density from air, light slows down and is bent from its original path which is known as refraction. This amount of refraction within a gemstone is known as its refractive index (RI).

Luster:

The overall appearance of a gemstone, which is its luster, is defined by the way light is reflected from its surface. Luster of a gemstone has a relation with the degree of surface polish, which increases with the increase in the hardness of a gemstone. The different types of luster are:

Adamantine: This is considered to be the top of the luster and is only found in “Diamond” because of its greatest hardness.
Vitreous: It is a “glass-like” luster, which can be seen in Ruby, Garnet, Etc.
Metallic: It is a “metal-like” luster, which can be seen in Pyrite crystals
Waxy: The best example of this kind of luster is Turquoise.
Greasy: This kind of luster can be seen in stones like “Jadeite”.
Resinous: This luster looks like a resin and “Amber” is considered to be the most appropriate example of this luster.
Silky: This “silk-like” luster can be seen in satin spar gypsum, which shows the best example of this kind of luster.