Optical phenomenon exhibited on certain minerals where a light ray enters the crystal and splits up into two separate rays. 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 displayed. The double refraction is different in every mineral, and can be used to identify gemstones. Double refraction is measured with a refractometer.
Double refraction in some level is exhibited on all non-opaque, non-amorphous minerals that do not crystallize in the isometric