What is Gemstone Birefringence?


Summary
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.
Reading time: 2 min 54 sec
gemstone birefringence - zircon
Natural zircon (not to be confused with cubic zirconia) has been used to create convincing diamond imitations. However, this gem is doubly refractive and has a high birefringence. Diamonds have no birefringence. 2.35-ct radiant step emerald-cut zircon. © Dan Stairs Custom Gemstones. Used with permission.

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.

Some gemstones have ranges for RI values for each of their axes. For example, microcline, a variety of feldspar, has the following RI values:

α axis = 1.514-1.529; β axis = 1.518-1.533; and γ axis = 1.521-1.539.

In cases like these, you calculate the birefringence as a range. You take the maximum difference of the smallest values and the maximum difference of the highest values. Thus:

1.521 – 1.514 = 0.007

1.539 – 1.529 = 0.010

So, the birefringence of microcline is 0.007-0.010.

microcline - gemstone birefringence
Feldspar, variety microcline. Photo by Randolph Black. Public Domain.

Birefringent Effects

Pleochroism

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.

Zoisite-20888
Zoisite (Var. Tanzanite), 8.5 x 4.0 x 2.0 cm, Merelani Hills (Mererani), Lelatema Mts, Arusha Region, Tanzania. © Rob Lavinsky, www.iRocks.com. Used with permission.

Fuzziness

Other effects can include a fuzzy, out-of-focus appearance, such as in this piece of adamite.

Adamite-39492
The fuzziness in this adamite isn’t a camera effect. Instead, the gem’s birefringence causes this naturally. Adamite (Var: Manganoan Adamite), 3.0 x 1.5 x 1.0 cm, Ojuela Mine, Mapimi, Mun. de Mapimi, Durango, Mexico. © Rob Lavinsky, www.iRocks.com. Used with permission.

Double Images

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.

2756010517_d612f79cca_o
Calcite. Photo by Anders Sandberg. Licensed under CC By 2.0.