What Are The Best Rough Sapphire Stones?


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“Corundum Var. Sapphire.” © Rob Lavinsky, www.iRocks.com. Used with permission.

Question

I’m interested in finding some nice sapphire stones. From what part of the world, I don’t know. What are the best rough sapphire stones? Songea, Tunduru, Umba, Australian, Montana … it’s all very confusing.

Answer

That is a big question, one that needs a full-length article to explain. However, here are some quick comments.

If you’re looking for a blue, the gems from Montana are usually “steely,” meaning they have a gray tone to them. That’s why they usually cost less than other blues. The value comes from the purity of color. “Cornflower” is greatly over used. If you’re a gardener, you know what the color refers to. Otherwise, I would best describe it as the deepest blue in a clear sky. This is the most valued color in sapphire. Lighter stones lose value quickly, deeper blues less so. Deep, royal blue gems are quite valuable, until they get too dark.

As a general rule, the best blue sapphire stones come from Southeast Asia, although the African ones are sometimes of pretty high quality.

For fancy colored sapphires, no one location stands out. Here, value has more to do with personal preference, as just about all shades of color are available and worth about the same. Hot pink sapphire, which is in high demand, is an exception.

If you get blue sapphires for faceting, you’ll find the color isn’t usually consistent all the way through. Dichroism, with one direction green, is common. Those gems must be oriented for color, regardless of the yield. Most sapphire stones also show zoning, for which you’ll need to account. It takes a lot of experience to adjust for both dichroism and zoning.

I cut one exceptionally satisfying sapphire. It was a Montana gem with a very fine, deep royal blue spot, not more than 2 mm in diameter. I got that spot in the culet and the gem was both richly colored and lively. In most deep blue gems, the color absorbs the light, so they aren’t very lively. Since this gem was mostly colorless, it had great sparkle in relationship to the color. That’s a good trick that you may be able to apply to a few sapphire stones that cross your path.

Donald Clark, CSM IMG

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Sometimes gems form naturally in a non-continuous manner. For example, a fracture in the Earth’s crust that was filled with the chemicals to create a particular gem may become exposed, stopping the stone’s growth. The fracture may be resealed, however, and the gem may continue to grow. If the same chemical mix is present, the layers in the gem created by this start-and-stop growth may be undetectable. If the fracture is resealed but contains a slightly different chemical mix, the layers formed may be distinguished by their different colors, or color zoning. Sapphire and rubies are varieties of corundum, crystalline aluminum oxide. Add a trace of chromium to aluminum oxide and you may grow a ruby. This gem features color zoning because it’s part sapphire and part ruby. “Sapphire/Ruby.” © Rob Lavinsky, www.iRocks.com. Used with permission.