Birefringence
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Definition
The difference between a gemstone's highest and lowest refractive indices, observed in anisotropic (non-cubic) crystals where light is split into two rays traveling at different speeds.
Detailed Explanation
Birefringence causes a visual doubling of back facets when viewed through the table of a stone, most dramatically seen in calcite (Iceland spar) and zircon. Calcite has an extreme birefringence of 0.172, while diamond — being isotropic — has zero birefringence. This property is an important diagnostic tool: a stone showing strong doubling of inclusions is unlikely to be glass or cubic zirconia.