Scratching another stone with a suspect diamond is an excellent test of authenticity. Scratching determines the hardness of the stone, which is a key identifying characteristic for diamonds.
A raw diamond looks similar to the expertly cut stones adorning necklaces and rings. At first glance, a rough diamond resembles common quartz or calcite. Diamonds are famous for their hardness, however, and this characteristic is easy to test. The Mohs hardness scale is a system of ranking gemstones based on hardness. The scale ranges from one to ten, with one being the softest and ten being the hardest. Diamond ranks as a ten on the Mohs scale, making it the hardest mineral. Scratching glass with a suspected diamond is a highly inaccurate test of authenticity, as glass ranks at approximately five or six on the Mohs scale; even quartz, with a hardness of seven, scratches glass. The best test of hardness for a diamond is to choose a stone that ranks at a hardness of nine because diamond is the only mineral capable of scratching a stone of this level. Corundrum, such as ruby or sapphire, ranks at a hardness of nine. If a piece of old or unwanted sapphire or ruby jewelry is available, it is an excellent tool for testing a possible diamond.