Chrysoberyl is a mineral with a hardness of 9 out of 10 on the Mohs scale of mineral hardness [?]. These Orthorhombicly structured gems are made of beryllium aluminum oxide, their full chemical compound being BeAl2O4.
Chrysoberyl appears as prismatic, tabular crystals often v-shaped twins forming pseudo-hexagonal crystals. Single crystals are rare.
May be colorless, green, yellow, gray or brown.
A variety, alexandrite, is red in tungsten light but dark green in daylight. This pronounced color change is highly-prized, and the exact tone of colors is important, the ideal being brilliant green turning to fiery red. When the colors are dullish, the value falls appreciably.
The cat's-eye variety is caused by microscopic inclusions of a lustrous reddish-brown mineral (rutile).
The true chrysoberyl is also known as "golden chrysoberyl" and is the most common form, appearing in various shades of yellow. Any cut may be used to set off the excellent luster of the stone.
Its appearance may range from transparent to translucent and is very hard, insoluble, heavy, yet fragile. It forms in many rocks, including pegmatites, schists, gneisses, and marbles. It also occurs in placer sands, which are alluvial deposits, and is resistant to weathering and erosion.
Alexandrite is found at Takowaja in the the Urals, and in Sri Lanka. Cat's-eyes are found in alluvial deposits in Brazil and Sri Lanka. Beautiful transparent or translucent yellow-green twins occur at Espirito Santo (Brazil).
Takes its name from the Greek "chryso" for "golden".
The alexandrite is an extremely rare gemstone of fairly recent history, and alexandrite owes its name to the fact that is was first discovered in the Urals in 1830, on the day of Prince Alexander of Russia's coming of age. Alexandrite is so rare that few people have ever seen one.
In the last few years, small quantities of bluish-colored garnet that turn red have been found in East Africa, but is singly refractive, unlike alexandrite.
The colorless varieties, alexandrite, and honey-yellow cat's-eyes are highly-prized gemstones. Fine stones are extremely valuable and on a par with emeralds, rubies, and sapphires.
The name alexandrite is usually applied to other mainly synthetic stones not remotely like it (usually violet-colored, synthetic corundums.) Attempts have been made to imitate alexandrite's change in color, but the results have been modest.
The more common golden chrysoberyl however, is valued less and under-appreciated, perhaps because of its weak color.