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A diamond's 'carats' are a weighty matter

I heard on the news they found a big blue diamond. They said it was something like 120 carats, and as they were talking about it, I realized I don't know what that means.

You gurus are good at these sorts of things, so tell me, when a jeweler or whoever starts talks about diamonds in carats, what are they actually saying?

— Andy, via phone

You're referring to that 122.52 carat blue diamond (shown) found near Pretoria, South Africa, last week by mining firm Petra Diamonds. When it goes to auction next month, analysts expect it to fetch more than the previous record for an uncut diamond of $35.3 million.

The jewelry department at Since You Asked headquarters could tell us only that rare blue diamonds go in the ocean at the end, so instead we reached out to Gail Stroud, certified gemologist at Lawrence's Jewelers in Medford.

In simplest terms, Stroud explained, carat is a unit of weight used to describe gemstones. In less simple terms, well, there are complexities in gemology.

"It doesn't relate to size, except that there's a standard," Stroud explained.

She said diamonds are typically 61/2; millimeters per carat.

"If it deviates from that, there's something wrong," she said.

According to Zales, the international standard since 1913 has been 200 milligrams per carat.

Stroud explained that there are different paths to diamond cutting, either maximizing the diamond's weight or maximizing the quality of the cut.

Stroud anticipates a middle ground for whoever cuts that big diamond.

"They're going to try to balance weight and beauty," she said. "They're not going to keep cutting and cutting until it's the perfect handler of light. ... It's extraordinary," she said.

Send questions to "Since You Asked," Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by email to youasked@mailtribune.com. We're sorry, but the volume of questions received prevents us from answering all of them.