The slowing down of light as it passes through a substance is measured in a number called the index of refraction. Interference colors are an effect produced by the fact that most solids have more than one index of refraction. The greater the difference between the indices of refraction, the more intense are the interference colors produced.
The difference between indices of refraction is called the birefringence. Following is a range of effects produced as the birefringence increases.
No Birefringence (Garnet) - appears black
(Quartz) - grays and whites
(Kyanite) - dull yellows, oranges, and browns
(Muscovite) - bright pastel-like colors - pinks, blues, greens
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