Double refraction - Phenomenon exhibited on all non-opaque minerals except for amorphous ones and ones that crystallize in the isometric system. A light ray enters the crystal and splits up into two separate rays, making anything observed through the crystal appear as double. The double refraction on most minerals is so weak that it cannot be observed without special instruments. However, in some minerals, such as the Iceland Spar variety of Calcite it is strongly seen. The double refraction is different in every mineral, and thus can be used to identify gems. Double refraction is measured with a refractometer