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'Very nice. But are they real?" My friend Kate was interrogating another friend, Sasha, about the glittering row of diamonds on her ring, a present from Sasha's husband on their 10th anniversary. It wasn't the most appropriate thing to ask, but Kate is about to split from her own husband and suspects that her engagement ring sports a rhinestone instead of a rock. "Something he said once," she says. And Bernice? Bernice these days is "all about CZ bling-bling" -- cubic zirconium. She's got herself a pair of big square diamond-stud CZ earrings and a pendant necklace to match. She dares anyone to tell the difference.

I sought some expert advice and found Steve Policastro, a Gemological Institute of America-certified gemologist and owner of the Southern California Gem Lab (619-236-1643) in downtown San Diego. Policastro specializes in identifying and evaluating people's gemstones. I started by asking him about the real thing.

"Diamonds are found primarily in South Africa, Canada, Australia, and Russia. They generally come from deep underground, and are formed by time, pressure, and heat. They're very expensive for a couple of reasons. First, there's a lot of labor involved -- they have to be mined; then they have to be sorted according to size and quality. Then they have to be cut and sorted again."

Quality is determined by several factors. "When you're looking for a quality diamond, you're looking for high clarity -- meaning you want few flaws in the diamond, few internal imperfections. There could be garnet inclusions. Generally, though, 'imperfections' refers to something done by Mother Nature during the pressure-and-heating process. Most diamonds have flaws. Flawless diamonds are very, very rare, and that's another reason they're so expensive."

Quality is also determined by color, graded on a descending scale from D to Z. A D diamond will be perfectly clear; as you go down the scale, you get an increasing degree of yellow. "The public starts to see yellow generally in the I-J-K range, but there are differences at every degree." Cutting provides the final quality factor; hence the re-sorting.

Mother Nature takes her sweet time in making a diamond -- about a thousand years, by Policastro's estimate. But now, he said, "through machines, we can intensify heat and pressure, and make a diamond in days. These are called synthetic diamonds. They haven't perfected the process to the point where they're bringing out flawless diamonds, but they are bringing out lower-quality stones and fancy colored diamonds. Synthetics are not too popular yet; one reason is that the cost is still fairly high compared to a natural stone. People can still buy a natural stone for only 20 percent more, but as more companies get into the synthesizing of diamonds, their price may come down. That may lead people away from the natural diamond market."

After all, said Policastro, a layperson can't tell the difference between natural and synthetic. "Even for gemologists, it's difficult. There is special equipment out now -- being sold for $10,000 to $20,000 -- that can tell the difference" by detecting differences in the spatial distribution of nitrogen defects in natural and synthetic diamonds.

But if a synthetic diamond is grown in a lab, it is still physically a diamond. "It has the same chemical, physical, and optical properties" as what gets dug out of the ground. Cubic zirconium is also lab-grown, but "it's a totally different stone, and it doesn't have all the same attributes as a diamond. It's an imitation, as opposed to a synthetic." It's got the looks and the sparkle, but "it will scratch much more easily, and it will wear down at the facet junctures. You'll start to see abrasions and chips on a cubic zirconium much more quickly than on a diamond. And while a diamond will retain some brilliance even when a ring accumulates something like soap scum, a cubic zirconium will lose its brilliance."

Policastro said he could tell the difference between diamond and cubic zirconium just by looking, thanks to some 20 years of looking at stones. But if there was any doubt, he could check the stone's specific gravity. "A cubic zirconium weighs almost twice as much as a diamond. A diamond with a 6.5mm diameter should weigh close to one carat; a 6.5mm cubic zirconium would weigh 1.85 carats. That's one positive test, but the stones need to be loose for that. There are also scratch tests and thermal conductivity tests."

Last, we came to rhinestones, the cubic zirconium of yesteryear. "Rhinestones are rock crystal quartz -- it's a white gemstone that they cut into costume jewelry many years ago. The primary purpose of rhinestones was to imitate diamonds, but the cubic zirconium is a better stone for that." Policastro judged the rhinestone the easiest impostor to spot. Like the cubic zirconium, the rhinestone is not as hard as a diamond, and suffers abrasions more easily. But the cubic zirconium "shows more brilliance. The rhinestone looks different internally; it has to do with the light refraction and reflection inside the stone." Not only that, it's more expensive. "There's a huge range in diamond quality, and so there's a huge range in price. But if you take an average 6.5mm diamond that costs $5000 wholesale, the same size rock crystal quartz would cost $3 to $4 , while the same size cubic zirconium would cost $.50 to $.75 ."

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