Written by Andrew Alden, updated September 5, 2019
Gemstones are more than just shiny, colored stones. Some of them also have certain optical "special effects." Most deal with surprising ways the stones play with light, including the fire and schiller effects.
These special effects, which are inherent in the mineral, are called "phenomena" by gemologists.
Skillful gem-cutting and techniques of the jewelry designer can bring out these special effects to their fullest, when desirable, or hide them when undesirable.
The special effect called fire by diamond cutters is due to dispersion, the ability of the stone to draw light apart into its constituent colors. This works just like the glass prism that unfolds sunlight into the rainbow by refraction.
The fire of a diamond refers to the coloration of its bright highlights. Of the major gemstone minerals, only diamond and zircon have strong enough refractive properties to produce distinct fire, but other stones such as benitoite and sphalerite show it, too.
Schiller is also known as play of color, in which the interior of a stone displays flickers of color as it is moved in the light. Opal is especially valued for this trait.
There is no actual object inside the stone. This special effect arises from light interference within the microstructure of the mineral.
Fluorescence is the ability of a mineral to turn incoming light of ultraviolet color into light of a visible color. The special effect is familiar if you've ever played in the dark with a black light.
Many diamonds have a blue fluorescence that can make a pale yellow stone look whiter, which is desirable. Some Southeast Asian rubies (corundum) fluoresce red, giving their color an extra glowing redness and accounting for the high price of the best Burmese stones.
Labradorite has become a popular stone because of this special effect, a dramatic flash of blue and golden color as the stone is moved in the light. It arises from light interference within microscopically thin layers of twinned crystals. The sizes and orientations of these twin lamellae are consistent in this feldspar mineral, thus the colors are limited and strongly directional.
05: Change of Color
Certain tourmalines and the gemstone alexandrite absorb certain wavelengths of light so strongly that in sunlight and indoor light they appear different colors. Change of color is not the same as the changes in color with crystal orientation that affects tourmaline and iolite, which are due to the optical property called pleochroism.
Iridescence refers to all sorts of rainbow effects, and, in fact, schiller and labradorescence can be considered varieties of iridescence. It is most familiar in mother-of-pearl, but it is also found in fire agate and some obsidian as well as many artificial gems and jewelry.
Iridescence arises from the self-interference of light in microscopically thin layers of material. A notable example occurs in a mineral that's not a gemstone: bornite.
Opalescence is also called adularescence and milkiness in other minerals. The cause is the same in all: subtle iridescence caused by scattering of light within the stone by thin microcrystalline layers. It can be a white haziness or soft coloration. Opal, moonstone (adularia), agate and milky quartz are the gemstones best known for this special effect.
Inclusions in a gemstone are usually considered flaws. But in the right kind and size, inclusions create internal sparkles, particularly in quartz (aventurine) where the special effect is called aventurescence. Thousands of tiny flakes of mica or hematite can turn plain quartz into a glittering rarity or feldspar into sunstone.
When impurity minerals occur in fibers, they give gemstones a silky appearance. When the fibers line up along one of the crystalline axes, a stone can be cut to display a bright reflective line a special effect called cat's-eye. "Chatoyance" is French for cat's-eye.
The most common cats-eye gemstone is quartz, with traces of the fibrous mineral crocidolite (as seen in tiger iron). The version in chrysoberyl is the most precious and is called simply cats-eye.
When fibrous inclusions align on all of the crystal axes, the cats-eye effect can appear in two or three directions at once. Such a stone, cut properly in a high dome, displays the special effect called asterism.
Star sapphire (corundum) is the best-known gemstone with asterism, but other minerals occasionally show it, too.Credit: Alden, Andrew. "Top Gemstone Special Effects." ThoughtCo., Dotdash, 5 Sept. 2019, https://www.thoughtco.com/top-gemstone-special-effects-1440586.