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Refractive Index

The refractive index of a substance is the degree to which light is bent as it passes through the substance. To demonstrate this to yourself, put a spoon into a glass of water and you will see that at the surface of the water the spoon handle doesn't join up! The water has bent the image (Water's RI is 1.33). This is exaclty what happens to light in a gemstone and very usefully all stones have their own refractive signature value when put into an instrument called a refractometer.

Most coloured gems have a range somewhere in the 1.43- 1.98 region. Diamond RI is 2.42 which means they bend light 2.4 times more than air does. This also means that light travels 2.4 times slower in diamond. As a general rule the higher the RI the greater the potential brilliance of the stone but cut and clarity also affect this.

There is another important issue with gemstones and refractivity, that is whether the stone is doubly or singly refractive, this depends on the crystal structure of the gem itself. Cubic composition means the stone is singly refractive (as in Diamond). With doubly refractive stones the light ray is split into two and each ray has a different angle, ruby is an example of doubly refractive stone. This is an important diagnostic test for stones.

Typical refractive index scores for common stones:

Haematite 2.94

Diamond 2.4

Cubic Zirconia 2.17

Zircon 1.9

YAG 1.8

Alexandrite 1.7

Coral 1.6

Emerald 1.5

Glass 1.4

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