Birthstone of the Month

By Will Postell - “Rock Reporter”

June affords you an excellent opportunity to claim three fantastic birthstones with that being pearl, moonstone, or alexandrite. We will focus on alexandrite for this month. Alexandrite (Al2BeO4) as a variety of chrysoberyl is an aluminum beryllium oxide. The most famous and highly prized variety of chrysoberyl was first discovered in the Ural Mountains of Russia only as recently as 1830 on the 21st birthday of Czar Nicholas Alexander II. Natural alexandrite is a classic example
of what the gem world calls the best-known colorchange phenomenon, or "chameleon" among gems. Alexandrite can be a vivid green emerald color by daylight
when viewed in sunlight, a lovely reddish purple in nightlight, or a bright red in incandescent light. Only occasionally will it display a chatoyance effect (“cat’s eye”).
Consider how "rare and costly" the high quality natural alexandrite is and you will find it has a "professional athletes only" price range as it is now considered one of
the most valuable gemstones.

Alexandrite, often containing small, needle-like parallel inclusions, is colored by chromium occurring in combination with iron. Alexandrite is rated an 8.5 on the
Mohs Scale of Hardness and 3¾ times as heavy as water. Alexandrite grows as tabular or prismatic crystals in granite pegmatites, metamorphic gneiss, mica schists, and is also found in alluvial deposits. The largest stone ever found, 1,876 carats, came from Sri Lanka. Large alexandrite stones are found these days only in museums or exclusive private collections.

Natural alexandrite being so rare and expensive has a much less costly alternative - a synthetic gem known as corundum alexandrite. The synthetic has been scientifically developed in the laboratory to duplicate a gem that occurs naturally. Its physical and chemical properties are virtually identical to the stone it appears to be. Since its introduction in the 1970s, corundum alexandrite has now become a favorite in the market place. Stones are faceted mainly in the step, Ceylon, round, brilliant cuts, and occasionally as an oval.

Today, alexandrite is mined in Sri Lanka, Zimbabwe, and since the end of the 1980’s especially in Minas Gerais in Brazil. Deposits are also found in Myanmar, Madagascar, and Tanzania. The future supply of alexandrite is at best uncertain. No important new sources have been found and current deposits are mostly sporadic.

… Will Postell

Sources: Gemstones of The World by Schumann; Rocks & Minerals by Jones; Gems by Dennis; Rocks & Minerals by Chesterman

Printed in June 2010 issue of Lowcountry Diggings