Birefringence and Double Refraction

Double refraction is the splitting of light into two rays as it passes through most crystalline gems. Each ray travels at its own speed, so the gem has two refractive indices. The difference between the highest and lowest of those indices is called birefringence. In gems with high birefringence, such as zircon and peridot, the split is large enough to see as doubled facet edges through the stone.

Isotropic and anisotropic materials

Gemologists sort materials into two groups by how they treat light.

Isotropic materials let light travel at the same speed in every direction. They are singly refractive and have one refractive index. This group includes gems in the cubic crystal system, such as diamond, spinel and garnet, and amorphous materials with no crystal structure, such as glass, opal and amber.

Anisotropic materials slow light by different amounts depending on direction. They split light into two rays and are doubly refractive. Every gem in the other six crystal systems belongs here, including corundum, beryl, quartz, tourmaline, topaz, peridot and zircon. The crystal systems page lists which gems fall in each system.

Calcite is the classic demonstration. Set a clear calcite rhomb on a printed line and you see two lines. Its birefringence is about 0.172, among the highest of any common mineral.

Uniaxial and biaxial gems

Doubly refractive gems are divided again by how many directions of single refraction they have. Along these directions, called optic axes, light does not split.

Uniaxial

Uniaxial gems have one optic axis. They belong to the tetragonal, hexagonal and trigonal systems. One of their two rays, the ordinary ray, has a constant refractive index in every direction. The other, the extraordinary ray, varies with direction. On a refractometer, one shadow edge stays still as the stone is rotated while the other moves. Corundum, beryl, quartz, tourmaline and zircon are uniaxial.

Biaxial

Biaxial gems have two optic axes. They belong to the orthorhombic, monoclinic and triclinic systems. Both refractometer shadow edges can move as the stone rotates. Topaz, peridot, chrysoberyl, tanzanite, iolite, kunzite and feldspars are biaxial.

Gemologists also describe a gem's optic sign, positive or negative, based on which index is higher. Quartz is uniaxial positive, while corundum is uniaxial negative. The optic sign and character together help confirm an identification.

Birefringence values

Birefringence is measured on a refractometer by recording the highest and lowest readings as the stone is turned. The difference is the birefringence.

Single refraction has a birefringence of zero. Diamond, spinel and garnet should show only one refractometer reading.

Facet doubling

When birefringence is high, the two rays travel through the stone on noticeably different paths. Looking through the table at the back facets, you see each facet edge twice. This is called doubling.

Doubling is easiest to see in zircon, peridot, titanite and synthetic moissanite. Use a 10x loupe and look at the pavilion facet junctions through a crown facet at an angle, since doubling disappears when you look straight down an optic axis. In zircon and moissanite, doubling is one of the easiest ways to separate them from diamond, which shows none. The moissanite page covers this test in more detail; moissanite is often cut with its optic axis perpendicular to the table to reduce visible doubling face-up.

Gems with low birefringence, such as corundum and beryl, rarely show visible doubling except in large stones viewed at an angle.

The polariscope

The polariscope tells singly refractive from doubly refractive gems quickly. It has two polarizing filters, one below the stone and one above, and a light source underneath. The filters are crossed so the field looks dark.

Place a gem between the filters and rotate it:

• A singly refractive gem stays dark through a full turn.
• A doubly refractive gem blinks light and dark four times in a full rotation.
• A polycrystalline material, such as chalcedony or jade, stays light the whole way around, because its many tiny crystals point in every direction.
• Glass and some singly refractive gems can show anomalous double refraction, a patchy or snakelike pattern caused by internal strain. Synthetic spinel and some garnets do this, and so can diamond.

With a conoscope (a small glass sphere or lens held above the stone), a gemologist can see an interference figure. A uniaxial gem shows a figure like a cross with rings, sometimes called a bull's eye. A biaxial gem shows one or two curved brushes. These figures confirm optic character when the refractometer cannot, such as on an over-the-limit stone.

Birefringence and pleochroism

Double refraction is also behind pleochroism, the change in color with viewing direction. Each of the two rays can be absorbed differently, so a colored doubly refractive gem may show two or three colors. The pleochroism page explains how cutters handle it. The gemological instruments page describes the polariscope, conoscope and dichroscope together.

Frequently asked questions

What is the difference between double refraction and birefringence?

Double refraction is the splitting of light into two rays. Birefringence is the number that measures it: the difference between the gem's highest and lowest refractive indices.

Why does zircon look fuzzy under a loupe?

Zircon has high birefringence, so you see each back facet edge twice. That doubling can make the stone look slightly fuzzy or out of focus up close.

Is diamond doubly refractive?

No. Diamond is cubic and singly refractive. Some diamonds show strain patterns in a polariscope, but they do not split light into two rays.

How can doubling help tell moissanite from diamond?

Synthetic moissanite has a birefringence of about 0.043, so a loupe shows doubled facet edges when viewed at an angle. Diamond shows none.