Gems that polarize light and split it into two (or three) different directions are said to be doubly refractive. Birefringence is a measure of a gem’s double refraction and it’s one of the principal ways gemologists can identify gems.
Light slows down (or bends) whenever it enters a gemstone. Gemologists calculate a gem’s refractive index, or RI, by dividing the speed of light in a vacuum by the speed of light as it passes through the gem. (Since the speed of light in a vacuum is always faster than the speed of light through any gemstone, an RI is always a number greater than 1). Gemologists use a device called a refractometer to measure a gem’s RI. Since the RI ranges of gemstones have been well established, this is a valuable gem identification technique.
Gems with cubic crystal structures, like diamonds, have only one RI since all the axes of its cubic structure are equal in length. They are not doubly refractive and have no birefringence. Amorphous gems like opals also have only one RI and no birefringence.
Gemstones with all other crystal structures are doubly refractive. They have two (or three) RIs based on the direction light enters them.
The difference between a gemstone’s highest and lowest refractive indexes is its birefringence number. The greater that number the more noticeable the effects of double refraction will be to the naked eye.
Other effects can be a fuzzy, out-of-focus appearance, such as in this piece of adamite.
Some gems are so birefringent that they may have a double vision effect. If the stone is facetted, the facets on the opposite side of the viewer will appear to be doubled. Some gemstones, such as calcite, will create a double image of whatever is placed behind it.