The mineral or gemstone chrysoberyl, is an entirely different gmineral to beryl with the two have greatly different properties. Chrysoberyl is the third-hardest natural gemstone with a hardness of 8.5. Chrysoberyl is a mineral consisting of ordinary colorless or yellow transparent chrysoberyl, cymophane (chrysoberyl cat’s eye), and alexandrite.
An interesting feature of its crystals are the cyclic twins called trillings. These twinned crystals have a hexagonal appearance, but are the result of a triplet of twins with each “twin” taking up 120 degrees of the cyclic trilling.
There are three main varieties of chrysoberyl: ordinary yellow chrysoberyl, cat’s eye or cymophane, and alexandrite. Although yellow chrysoberyl was referred to as chrysolite during the Victorian and Edwardian eras, that name is no longer used in the gemological nomenclature.
is a yellowish-green, transparent to translucent chrysoberyl and has often been referred to in the literature as chrysolite due to the common olive color of many of its gems, but that name is no longer used in the gemological nomenclature. When the mineral exhibits good pale green to yellow color and is transparent, then it is used as a gemstone.
a strongly pleochroic (trichroic) gem, will exhibit emerald green, red and orange-yellow colors and tend to change color in artificial light compared to daylight. The color change from red to green is due to strong absorption of light in the yellow and blue portions of the spectrum. Typically, alexandrite has an emerald-green color in daylight but exhibit a raspberry-red color in incandescent light.
is popularly known as cat’s eye. This variety exhibits pleasing chatoyant or opalescence that reminds one of an eye of a cat. When cut to produce a cabochon, the mineral forms a light-green specimen with a silky band of light extending across the surface of the stone.
Chrysoberyl is normally yellow, yellow-green, or brownish with its color being caused by the presence of iron. Spectroscopic analysis will usually reveal a strong band where the violet takes over from the blue. As the color darkens from bright yellowish-green to golden-yellow to brown, this band increases in strength. When the stone has a strong color, two additional bands can be seen in the green-blue. The most common inclusions are liquid-filled cavities containing three-phase inclusions. Stepped twin planes may be apparent in some cases. Some very rare minty bluish-green chrysoberyls from Tanzania owe their color to the presence of Vanadium.
Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones. Members of the beryl group include emerald, aquamarine, and morganite while members of the chrysoberyl group include chrysoberyl, cymophane (cat’s eye), and alexandrite. Beryl is a silicate and chrysoberyl is an oxide and although both beryl and chrysoberyl contain beryllium, they are separate gemstone species unrelated in any other way. Because of the confusion between chrysoberyl and beryl, chrysoberyl is relatively unknown in its own right and the alexandrite variety is much more widely recognized. The only well-known natural gemstones harder than chrysoberyl are corundum and diamond.
The alexandrite variety displays a color change (alexandrite effect) dependent upon light, along with strong pleochroism. Alexandrite results from small scale replacement of aluminium by chromium oxide, which is responsible for alexandrite’s characteristic green to red color change. Alexandrite from the Ural Mountains in Russia is green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light. The optimum or “ideal” color change would be fine emerald green to fine purplish red, but this is exceedingly rare. Because of their rarity and the color change capability, “ideal” alexandrite gems are some of the most expensive in the world.
The finest alexandrites up to 5 carat are being found in the Ural Mountains, but the largest cut stones are in the 30 carat range, though many fine examples have been discovered in Sri Lanka (up to 65 cts.), India (Andhra Pradesh), Brazil, Myanmar, and especially Zimbabwe (small stones usually under 1 carat (200 mg) but with intense color change). Overall, stones from any locale over 5 carats (1 g) would be considered extremely rare, especially gems with fine color change. Alexandrite is both hard and tough, making it very well suited to wear in jewelry.
The gem has given rise to the adjective “alexandritic”, meaning any transparent gem or material which shows a noted change in color between natural and incandescent light. Some other gem varieties of which alexandritic specimens have been found include sapphire, garnet, and spinel.
Some gemstones described as lab-grown (synthetic) alexandrite are actually corundum laced with trace elements (e.g., vanadium) or color-change spinel and are not actually chrysoberyl. As a result, they would be more accurately described as simulated alexandrite rather than synthetic but are often called Czochralski Alexandrite after the process that grows the crystals.
Synthetic alexandrite is used as an active laser medium. Alexandrite laser crystals tend to be round, with a pale brown tint.
Genuine alexandrite is one of the most expensive gemstones available commercially, with the stronger color changes being more highly valued.
Translucent yellowish chatoyant chrysoberyl is called cymophane or cat’s eye. Cymophane has its derivation also from the Greek words meaning ‘wave’ and ‘appearance’, in reference to the chatoyancy sometimes exhibited. In this variety, microscopic tubelike cavities or needlelike inclusions of rutile occur in an orientation parallel to the c-axis producing a chatoyant effect visible as a single ray of light passing across the crystal. This effect is best seen in gemstones cut in cabochon form perpendicular to the c-axis. The color in yellow chrysoberyl is due to Fe3+ impurities.
Although other minerals such as tourmaline, scapolite, corundum, spinel and quartz can form “cat’s eye” stones similar in appearance to cymophane, the jewelry industry designates these stones as “quartz cat’s eyes”, or “ruby cat’s eyes” and only chrysoberyl can be referred to as “cat’s eye” with no other designation.
Gems lacking the silky inclusions required to produce the cat’s eye effect are usually faceted. An alexandrite cat’s eye is a chrysoberyl cat’s eye that changes color. “Milk and honey” is a term commonly used to describe the color of the best cat’s eyes. The effect refers to the sharp milky ray of white light normally crossing the cabochon as a center line along its length and overlying the honey colored background. The honey color is considered to be top-grade by many gemologists but the lemon yellow colors are also popular and attractive. Cat’s eye material is found as a small percentage of the overall chrysoberyl production wherever chrysoberyl is found.
Cat’s eye really became popular by the end of the 19th century when the Duke of Connaught gave a ring with a cat’s eye as an engagement token, this was sufficient to make the stone more popular and increase its value greatly. Until that time, cat’s eye had predominantly been present in gem and mineral collections. The increased demand in turn created an intensified search for it in Ceylon. Early 20th century prices could go up as high as $8000 for a cut stone.