The property of splitting a single beam of light into two polarized beams which pass through a crystal at different speeds. All crystals, except those in the cubic system, possess this property. The difference between double refraction and birefringence is that double refraction is an optical effect (which can be observed) while birefringence is an intrinsic value. In daily use birefringence and double refraction are the same.
Zircon has a Very High Birefringence which is Clearly Visible in this Shot. The Back Facets Appear Doubled.
Image Courtesy of Jeffrey Hunt.