Written by rdjadmin On May 2, 2013.
At last count, gemologists had logged in more than 100 tints for tourmaline, making it the gem world’s king of color. Centuries ago, however, when its broad palette meant constant confusion with other stones, tourmaline was one of the gem world’s masters of disguise. Indeed, the gem’s root-name, is turmali, a Sinhalese term used by Ceylonese dealers and means “mixed gemstones of unproven identities.” Today the original name is still apt because it reflects a gemological diversity so great that the gem is classified as a group rather than a family.
Yet, strangely enough, the gem that comes in more colors than any other begins life as colorless, or nearly so. During formation, its crystal structure hungrily absorbs trace elements from its host solution. Some of these chemical impurities — most notably, manganese for pink and iron for green — act as coloring agents when they replace atoms normally present.
This susceptibility to color change through chemical substitution makes tourmaline what is called an “allochromatic” gem. Of course, tourmaline is far from alone in this trait. In fact, without such color-susceptibility, many revered gems would be missing from jewelry stores. If, for instance, chromium didn’t replace aluminum in beryl, there would be no emerald.
Nevertheless, tourmaline takes to a wider variety of tinting than any other gem. And because it does, explanations of color causes in tourmaline are still largely theoretical and often conflict with one another. When it comes to color, tourmaline seems to play by lots of rules. “In Brazil, the color chemistry for tourmaline changes from pocket to pocket within an occurrence,” gemologist Joel Arem notes, “while in California it remains fairly uniform throughout the whole San Diego gem belt.”
Many tourmalines provide a visual record of changes in their chemistry by displaying two, sometimes three, areas of different color along their crystals. When cut, these multicolor gems become what Arem calls “history books of tourmaline growth.” Most frequently seen with two large zones of adjoining color, the gem is best known as bicolor tourmaline and usually features combinations of pink and green. Both the pinks and greens run wide gamuts from magenta-purple to brownish-pink and aloe to granny-smith apple green, with numerous intermediate shades.
Occasionally, bicolors depart from pink-green pairings and couple spruce-greens with aqua-blues, aqua-blues with cherry-pinks, and grass-greens with pumpkin-oranges, to name a few of the most beautiful combinations. “No doubt, new finds will give us new color combinations,” Arem adds.
At present, bicolor tourmalines are mined principally in Brazil, Zambia, Madagascar, Nigeria, Afghanistan and California.
It would be nice to lay down some hard-and-fast rules for buyers of bicolor tourmaline. But taste with this gem is as wide as its color palette. And that’s how it should be. However, if you insist that stones be eye-clean, this will pretty much restrict you to California, whose Himalaya Mine in tourmaline-rich San Diego County is famous for flawless bicolors. This doesn’t mean San Diego is the one source of flawless stones. Most localities produce some, but nothing proportionate to California. No matter what the origin, flawless bicolor tourmaline will nearly always have lighter hues.
As for color composition, California’s tends to have soft yellow-greens reminiscent of spring grass and pastel purples, often with a touch of brown.
It is hard to generalize about Brazilian color. In addition to green and salmon-pink, some stones show a combination of green with a dusky brownish-purple. Occasionally, stones possess a lovely indicolite-blue that commands an extra premium for rarity.
Zambia is noted for striking grass-green/pumpkin-orange combinations and, once in a while, unique aloe-green/terra cotta pairings. Madagascar’s stones have duskier greens and pinks. Afghanistan’s stones tend toward mint green and pastel pink. Be on the lookout for rare periwinkle-blue/magenta team-ups. Finally, there’s Nigeria, a source known for pairings of the same color, usually light to dark green.
When selecting bicolor tourmaline, expect the ratio of the two colors to be at least 60/40- with 50/50 the ideal. Breaks between colors should be straight up and down with minimal transitions.
Bicolored tourmaline has always been a gem for custom jewelry only. The very things about the gem that turn on makers of one-of-a-kind jewelry turn off manufacturers of mass-production jewelry.
Strike one for manufacturers is the unusual shape.While other tourmalines come in cushion, oval and even round shapes, bicolored ones are typically cut only as emerald shapes, many of them extremely thin.