Who doesn’t love a one-of-a-kind gemstone? Among the most desirable gemstones are those with a unique optical effect. Gemologists refer to these beautiful specimens
as “phenomenal gemstones“ due to their exclusive and uncommon properties. Star Sapphires and Rubies
Star sapphires and star rubies are two precious stones that
have been coveted for centuries. Stones that exhibit a star-like phenomenon, star sapphires and star rubies have intersecting needle-like inclusions over their crystal structure, causing an effect known as asterism. These inclusions are due to the presence of a mineral called rutile. The raw stone is often cut into a cabochon shape to showcase the star-like phenomenon. Ideally, the “star” is directly in the center of the stone when viewed from above, giving it the unique brilliance that makes it so special
. A great deal of skill and technique is required to shape the beautiful stone into a cabochon. Star sapphires and rubies are guaranteed untreated gemstones, as heat treatments destroy the inclusions that cause asterism, thus losing the mystic star-like effect.
This 28.00-carat star sapphire offered by M.S. Rau is a gleaming example of the asterism phenomenon. Its high dome amplifies the effect, as it may be viewed directly in the center of the stone from above the wearer’s hand. Cat’s Eye
When gazing into the eyes of a feline, one may notice the animal’s pupils narrowing to a thin slit when exposed to bright light. The cat’s eye phenomena in gemstones, also known as chatoyance
, is an optical circumstance in which a band of reflected light, known as a “cat’s eye,” is exhibited just beneath the surface of a cabochon-cut gemstone. Stones that contain a large number of very thin and parallel inclusions, known as “silk,” will exhibit the cat’s eye phenomenon. These inclusions may be crystals, such as hematite and rutile, hollow tubes or other linear structures. The chatoyance effect may be understood by comparing it to the way light reflects and forms a line down the center of a pool of silk thread, hence the namesake for the stone’s parallel inclusions. Much like a cat looking into the sun, the stone’s inclusions reflect the light source as a thin band across its surface with dazzling effect.
These exquisite earrings for sale at M.S. Rau feature numerous Chrysoberyl cat’s eye cabochons amid sparkling round-cut diamonds. The phenomenon is fabulously showcased in these rare stones arranged in a floral motif. Color-Changing Gemstones
A lesser known yet intriguing phenomenon is color-changing gemstones. The most notable example of this is Alexandrite. This rare form of Chrysoberyl exhibits an intense color change when exposed to different lighting. The stone’s absorption of different wavelengths of light combined with varying trace elements present in its structure produce a color-changing effect. Alexandrite
Alexandrite is among the most popular color-changing gemstones. First discovered in the Ural Mountains of Russia in the 1830s, the stone was named after Tzar Alexander II of Russia. Upon their discovery, jewelry lovers knew they had found something special by the stone’s vivid tones and dramatic color change. The spectrum of colors an Alexandrite displays vary from green in the light of day and red, brown or even purple under an incandescent light source. Alexandrite not only displays a spectrum of colors in different kinds of light, but also when viewed on an angle. This phenomenon is due to the stone’s unique way of absorbing different wavelengths of light and is referred to as pleochroism
This marvelous color-changing Alexandrite ring offered by M.S. Rau is a dazzling example of the desired phenomenon. Pictured is the same ring under different light sources, showcasing just how drastically the hue can vary. Adularescence
Although not an easy word to remember or pronounce, the optical effect of an adularescent
stone is certain to stick in your memory. Often referred to as schiller
, the milky luster attributed to this phenomenon is most commonly observed in moonstones or opal stones. This phenomenon is reminiscent of lunar light reflecting off of water, accounting for moonstone’s name. The bluish glow or luster of an opal or moonstone is produced by an effect called adularia
, or the interaction between reflected light and micro-structures of the mineral below the stone’s surface. Because of these structures, opal birthstones give off a brilliance that makes each stone unique and breathtaking.
This necklace exhibits a string of stunning opals that showcase the lustrous effect of adularescence. The 89 Australian opals exhibited on this necklace glow from the inside out, a truly exemplary string of stones exhibiting this phenomenon.