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.