You've found a crystal that sparkles and shines, flashing light every time you look at it. While it may be visually arresting, it can be extremely difficult to tell if this luminous stone is a diamond or a quartz. Both stones are clear and can be polished to an extreme shine, but their chemical make-up and market value are entirely different. There are some tricks you can try from home to determine whether your stone is a diamond or a quartz, though the best method is to consult a professional jeweler.
Perform a simple scratch test to gauge the hardness of your stone. Diamonds rate a 10 on the Mohs scale of hardness, making diamonds the hardest mineral substance. A diamond can only be cut by other diamonds. Quartz rates a seven on the scale, which places it at the same hardness as a steel file.
Choose an unobtrusive spot on your crystal, such as the sides or bottom, and lightly scratch the crystal with a steel file. It does not have to be a large scratch or a deep one. If the crystal shows any impact, you are looking at quartz. Remember that topaz is harder than quartz, so just because your stone is not affected does not mean conclusively that it is a diamond.
Familiarize yourself with the names given to quartz stones that resemble diamonds. If someone is selling a Herkimer diamond ring, you are not getting a true diamond. You are purchasing a quartz crystal that may superficially resemble a diamond but it was actually mined in upstate New York. Other names of quartz "diamonds" include Stolberg, Lake County, Mirabeau, Bohemian, Carrera, Mamarosch and Schaumburg diamonds.
Subject your crystal to a thermal conductance test. Thermal conductivity measures how quickly and effectively a given object directs heat. Diamonds are excellent conductors of heat and energy, while quartz is relatively ineffective.
Thermal conductivity testers can be purchased from a variety of online jeweler supply and mineral enthusiast companies, and typically cost over $100.
Speak to a professional jeweler in your area to see if he can test your crystal with his own equipment. A fee may be associated with this service, but hopefully it will be less than the cost of purchasing equipment that you may rarely use.