"Diamonds are forever" must be one of the world's most successful advertising slogans, and a diamond is often described as the "perfect gift." Very well. Suppose that your boyfriend (or girlfriend, let's not be sexist here) brings you as a present a 20-carat diamond.

Even though it doesn't weigh very much (there are about 142 carats to an ounce, so 20 carats is only a seventh of an ounce), that's a large stone by jewelry standards. The biggest diamond ever found, the Cullinan diamond, weighed 22 ounces before being cut and shaped, but that was a true monster. Most people are quite happy to be given a one-carat diamond ring.

Rendered tactless by the generosity of the gift, you ask how much it cost. You are told, $100 - but that amazing price was possible, your friend explains, only because the stone was "hot." You wonder: Is it a real diamond, or is it a fake? It might be a zircon, or a colorless topaz, or even "strass," a glass, rich in lead, that makes the "paste" commonly used in stage jewelry.

How can you tell the fake from the real thing?

There are three or four methods. The first, and the easiest one to apply, is to examine the hardness. Gemstones need to be hard, otherwise the edges and corners of their faceted sides will wear down through the action of ordinary dust and grit, and their brilliance will quickly be lost.

Diamonds are famously hard; in fact, they are the hardest material found in nature. They are not forever, but they are the next best thing to it, resistant to erosion from contact with other materials. The only way you can grind diamonds to the glittering multi-faceted shapes that we see is with an abrasive paste made with "bort," finely ground and impure diamond dust. Diamonds are pure crystallized carbon, produced by nature, and recently by humans, under conditions of enormous temperature and pressure (thousands of degrees and millions of pounds per square inch). They are composed of exactly the same material as graphite, or soot. As Oscar Wilde remarked, "Appearances are everything."

We need some way to rate the hardness of diamonds. There is a convenient scale, known as the Mohs' scale, that has been in use for just such a purpose for two centuries (it was introduced in 1812). The Mohs' scale ranks substances by the ability of one of them to make a scratch in another, and it ranges from 1 (talc) to 10 (diamond). Everything else lies within this range, so that for gemstones, rubies and sapphires have a Mohs' index of 9, topaz is 8, emerald is 7.75, zircon is 7.5, garnet is 7.25, amethyst is 7, and opals lie between 5.5 and 6.5. Ordinary window glass is between 5 and 6. Your fingernail has a Mohs' index of about 2.5.

There also are more quantitative tests for hardness, yielding what are known as the Brinell index and the Knoop index. We won't go into them here, except to point out that the Brinell index, the Knoop index, and the Mohs' scale give compatible results, and the Knoop test can be applied in cases where the other scales cannot.

Examining the hardness values listed for different gemstones, you can see at once the problem with the old rule, "If it will scratch glass, it's a real diamond." That's simply not true. Lots of fake jewels, including topaz, ruby, sapphire, spinel, and zircon, easily scratch window glass. A better and much safer statement is the negative version: "If it won't scratch glass, it's not a real diamond."

If your friend's gift can't be used to write your initials on the window, forget it. Whatever that stone is, it is certainly not a diamond. But suppose it does scratch glass. What then?

We go to a second method for distinguishing real diamond from the various fakes: density. A real diamond has a density about 3.5 times that of water. Zircons are more dense, at 4.6 to 4.7, as are rubies and sapphires at 4.0 to 4.1, while emeralds are down at about 2.7. Because all gemstones are made from rather simple compounds, such as aluminum oxide, iron oxides, and zirconium oxide, their densities vary little between different specimens of the same stone. However, for the amateur, finding the density of a gemstone is quite a problem. The professional gemologist keeps on hand liquids that make the test quick and easy. For instance, methylene iodide has a density of 3.3. Drop your stone in, and a zircon, ruby, sapphire, or diamond will sink, while most other gems will float. Use methylene iodide saturated with iodine and iodoform, and the density goes up to 3.6. In this, a diamond will float but a zircon, ruby, or sapphire will sink. The most difficult case occurs when the stone you were given is a colorless topaz. The density is almost exactly the same as that of a diamond, and topaz is easily hard enough to scratch glass.

What next? Well, the fact that diamond is hard does not mean that it will not shatter if you hit it. A diamond will cleave or split easily, in four well-defined directions. Topaz will split in only one direction, and quartz or glass not easily in any consistent direction. The problem with this test is that when you are done with it, there may be not much left of the original stone.

The best test of all, again unfortunately not one you can easily conduct at home, is through the stone's ability to bend rays of light that enter it. The measure of this is called the refractive index, and it is higher for diamonds than for any other gem, which accounts for a diamond's brilliance. The refractive index of diamond is 2.43. Water, by comparison, has a refractive index of 1.33. The candidates for fake diamond run over a wide ascending range of refractive indices, from clear quartz at 1.45-1.55, crown glass between 1.5 and 1.6, topaz at 1.63, flint glass as high as 1.75, colorless sapphire at 1.77, and zircon at 1.97. The high value for zircon gives it a brilliant appearance and it is an excellent choice for a fake diamond, though it badly fails the density test.

However, you now have enough tests to be sure. The stone you were given is hard enough to mark glass and anything else you can find; it has a density much more than 3, and less than 4; and it has a refractive index well in excess of 2. That rules out zircons, all forms of glass or quartz, and colorless topazes and sapphires.

What you have is a genuine diamond. With decidedly mixed feelings, you turn the stone (and perhaps your generous friend, too) over to the police.

"Borderlands of Science" is syndicated by: