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. It serves as one of the principal ways gemologists can identify gems.
Measuring Refractive Index
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.
Which Gems Show Birefringence?
Gems with an isometric or cubic crystal system, like diamonds, have only one RI since all the axes of its cubic structure are equal in length. They aren’t doubly refractive and, thus, have no birefringence. Amorphous gems like opals also have only one RI and no birefringence.
Gemstones with all other crystal systems are doubly refractive. They have two (or three) RIs based on the direction light enters them.
How do You Calculate a Gem’s Birefringence?
The difference between a gemstone’s highest and lowest RIs is its birefringence number. The greater that number, the more noticeable the effects of double refraction will be to the naked eye.
If gemstones with high birefringence have color, they may display pleochroism, two or three colors, such as naturally trichroic zoisite. You can see different pleochroic colors depending on your viewing angle.
Other effects can include a fuzzy, out-of-focus appearance, such as in this piece of adamite.
Some gems are so birefringent that they create a double vision effect. If the stone is faceted, 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 lies behind it.