Gemstone Optics

Gemstone optics is the study of how gems reflect, bend, split and scatter light. A gem's beauty depends on more than its color. Luster describes how its surface shines. Brilliance and fire come from light moving through the stone. A smaller group of gems shows phenomena, such as the star in a star sapphire or the shifting glow of moonstone, caused by inclusions or internal structures.

Luster

Luster is the quality of light reflected from a gem's surface. It depends mainly on refractive index and on how well the surface takes a polish. Gemologists use a set of descriptive terms:

High refractive index gives high luster. Diamond, with an RI of 2.42, has adamantine luster. Quartz, at about 1.54, is vitreous. The refractive index page explains how RI is measured.

Brilliance

Brilliance is the white light returned to the eye from inside and on the surface of a gem. When a stone is well cut, light enters through the crown, reflects off the pavilion facets and comes back out through the top. Light that strikes an inner facet at too steep an angle passes straight out the back and is lost.

Two things control brilliance. The first is refractive index: high-RI gems bend light more and reflect more of it back inside, so a cutter has more room to return light. The second is cut. Proportions that are too shallow or too deep let light leak out, creating dark areas or a "window" in the center. The diamond optical properties page covers how this works in diamond.

Dispersion

Dispersion is the splitting of white light into its spectral colors. Each wavelength bends by a slightly different amount as it passes through the stone, so violet separates from red. In a cut gem, these colors flash as small points of colored light, called fire.

Dispersion is measured as the difference in refractive index between two set wavelengths in the red and violet. Some typical values:

Body color can mask fire. A dark green demantoid may show less fire than a pale one despite the same dispersion. Synthetic moissanite has more than twice the dispersion of diamond, and its strong fire is one way to tell them apart. The moissanite page covers this. Titanite also shows strong fire.

Scintillation

Scintillation is the pattern of light and dark flashes seen when a gem, the light or the viewer moves. It depends on the number, size and arrangement of facets. A brilliant cut produces many small flashes, while a step cut produces fewer, larger ones.

Optical phenomena

Phenomena are special optical effects caused by inclusions or by the structure of the stone. Most phenomenal gems are cut as cabochons because a curved surface concentrates the effect.

Asterism

Asterism is a star of light that seems to glide across the surface of a cabochon. It is caused by sets of fine, parallel needle-like inclusions, usually rutile, oriented in different directions. In corundum, three sets of needles at 60 degrees produce a six-rayed star. Star sapphire and star ruby are the classic examples. Some garnets show four- or six-rayed stars, and star diopside shows four rays. Twelve-rayed stars occur rarely in corundum.

Chatoyancy

Chatoyancy, or the cat's eye effect, is a single bright band of light across a cabochon. One set of parallel fibers or needles reflects light into a line that moves as the stone turns. The finest cat's eyes are chrysoberyl, which gemologists refer to simply as "cat's eye." Quartz, tourmaline and tiger's eye are also chatoyant.

Adularescence

Adularescence is a soft, billowy glow that seems to float just below the surface of a stone. It comes from thin, alternating layers of two feldspars, orthoclase and albite, that scatter light. Moonstone shows the effect best. A bluish glow is the most prized.

Labradorescence

Labradorescence is a play of metallic colors, often blue, green, gold and orange, that flashes across labradorite when viewed at certain angles. It comes from light interference in very thin layers within the feldspar. Spectrolite, from Finland, shows a full range of colors. The labradorite page explains more.

Play of color

Play of color is the patchwork of spectral colors in precious opal. It is caused by diffraction of light through orderly arrays of tiny silica spheres. Larger spheres can diffract red light, while smaller ones only produce blues and greens.

Iridescence

Iridescence is a rainbow-like effect caused by interference from thin films or layers. It appears in pearl orient, fire agate, some coated stones and iris quartz with internal fractures.

Aventurescence

Aventurescence is a glittery sparkle caused by small, flat, reflective inclusions. In aventurine quartz, the platelets are often green fuchsite mica. In sunstone, they are hematite or copper platelets. Aventurine glass, sometimes called goldstone, imitates the effect with copper crystals.

Color change

Color change is a change in color with different light sources. Alexandrite is green in daylight and red in incandescent light. The color in gemstones page explains why.

Frequently asked questions

What is the difference between brilliance and fire?

Brilliance is white light returned from the gem. Fire is the flashes of spectral color caused by dispersion, the splitting of white light into its colors.

What causes the star in a star sapphire?

Fine rutile needles aligned in three directions within the corundum crystal reflect light into a six-rayed star. The gem must be cut as a cabochon to show it.

Which gem has more fire than diamond?

Synthetic moissanite, demantoid garnet, titanite and cubic zirconia all have higher dispersion than diamond and can show more fire.

What gemstone shows a cat's eye?

Chrysoberyl gives the finest cat's eyes, but quartz, tourmaline, apatite and tiger's eye can also show the effect.