What is Gemstone Birefringence?

How Do You Measure a Gem's Refractive Index?

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. Light slows down (or bends) whenever it enters a gemstone. 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?

Gemstones that form in any crystal system except the isometric or amorphous are birefringent. They have two (or three) RIs based on the direction light enters them. The crystal systems that produce gems with birefringence are the tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and trigonal.

On the other hand, gems that form in the isometric or cubic crystal system, like diamonds and garnets, have only one RI since these crystals don't polarize light. They aren't doubly refractive and, thus, have no birefringence. Amorphous gems like opals also have only one RI and no birefringence.

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 any birefringent effects will be to the naked eye. The lower the number, the less noticeable any birefringent effects will appear.

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.

What are 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. In contrast, colored gemstones with very low birefringence may not show pleochroism. The differences in color may be too slight to be perceived by the human eye.

Fuzziness

Some doubly refractive gems may have a fuzzy, out-of-focus appearance, such as this piece of adamite.

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.