Finding Gem Characteristics

The gemologist often is handicapped in trying to determine gem characteristics. If the stone is mounted in some kind of jewelry setting, manipulation for tests is difficult. Owners frequently object to removing stones from their mountings since damage is possible. Even if the stone is not mounted, it is not practical to risk ruining the cut or polish by scratching, chipping, or removing a piece for testing. Every bit of damage reduces the value of the gem. Testing on cut stones is usually limited to nondestructive manipulation. Unfortunately, some of the best gem study techniques involve destruction of the sample. Destructive tests are carried out, then, on uncut gemstones. Among such tests are chemical analysis, X-ray structure determination, and Mohs' tests for hardness. Nondestructive tests include determination of refractive index, specific gravity, pleochroism, spectral pattern, and examination for any foreign inclusions in the stone.

Hardness

As already mentioned, hardness testing is destructive or damaging to the stone. It is determined by actually trying to scratch the stone with some of the minerals in Mohs' scale of hardness. Bits of the harder minerals in the scale—from 5 (apatite) to 10 (diamond)—can be obtained already mounted in small metal rods for convenient manipulation. Carefully, an attempt is made to scratch the gem with one of these hardness pencils, perhaps #7. The scratching is done under magnification and along the edge of the gem where it will not mar any of the facets. Only a tiny, almost invisible scratch is necessary. If #7 will not produce a scratch, #8 is tried. Should #8 produce a scratch then it is obvious that the gem's hardness lies between $1 and #8. Estimates can be made about whether it is 714, 71/2, or 7%, depending on how easily #8 made the scratch.