Chrysoberyl is composed of beryllium aluminum oxide and is colorless when in its pure form. Trace amounts of iron color the mineral yellow (as seen here), brown, and green. Chrysoberyl is an extremely hard and durable mineral. It is most commonly found as greenish-yellow crystals. Although better known for the rare and valuable cat's eye and alexandrite varieties, chrysoberyl is also cut into such attractive gems as this 114.25 carat stone from Minas Gerais, Brazil. Chrysoberyl gems were popular in 18th and 19th century Portuguese and Spanish jewelry and in Victorian England, but today are not widely seen in jewelry. The name Chrysoberyl comes from the Greek word "chrysos," meaning golden, and the mineral beryl. It was thought to be a type of beryl until the late 18th century, when it was recognized as a distinct mineral.
One of the most valuable gemstones is the variety of chrysoberyl known as alexandrite. Alexandrite is renowned for its color change from red under incandescent light, to green in daylight or fluorescent light. Alexandrite was discovered in 1830 in the Ural Mountains of Russia and named after Csar Alexander II. Sri Lanka and Brazil are the leading producers of alexandrite, but the finest quality color change gems have typically come from Russia. Photographed here under incandescent light is a 65.08 carat square-cushion cut alexandrite from Sri Lanka; a 16.69 carat cushion cut alexandrite from Sri Lanka; and a 4.84 carat emerald cut alexandrite from Russia.
With its exceptional size, honey color, and sharp band of light, the 58.19 carat Maharani Cat's Eye from Sri Lanka is one of the finest gems of its kind. The phenomenon of chatoyancy can be displayed by many gemstones, but the most popular and highly prized is that of the mineral chrysoberyl. In fact, the term "Cat's Eye" is synonymous with this stone, owing to the resemblance of the phenomenon with the slits of cat's pupils. The "eye" that the stone displays when it is cut en cabochon is caused by the reflection of light off numerous parallel inclusions of fine, needle-like crystals, commonly of the mineral rutile.