Just in case a diamond purchase looks too good to be real it is best to check that the gem is not fake.

• Diamonds disperse heat far more rapidly than most substances. If you breathe on a diamond it will not fog up, or at least not for very long. This is best noticed by comparing a genuine diamond to a glass replica. Breath on both, and the diamonds will almost instantly de-fog, while the glass will take at least a few moments to clear up. If prospective gemstone does not fog up it is a pretty good sign that it is not glass, though it might be something else.
• Mined diamonds almost always have flaws. If there is no imperfections visible under a jewellers loupe (magnifying glass for the eye) the items is probably not natural. Synthetic diamonds, made in a laboratory, are quite free from imperfections, are an issue with this test. They are 100% genuine diamonds, but did not form naturally. They still have about two thirds the value of a naturally occurring diamond.
• Look at refractivity. If you can see anything through the gem it is probably not real. Diamonds bend light so sharply that even something pressed against the surface is not visible on the opposing surface. You cannot read printed material though a loose diamond, or even make out a dot.
• Diamonds sparkle and refract light, but they do not alter the colour. Light through a diamond it white and grey. If you see a rainbow the gem is not a diamond.
• A diamond will sink in water; some other substances will not
• Many fake substances will crack when heated and quickly cooled. A diamond will not be prone to this issue.
• One substitute for diamonds is the Cubic zirconium. These look very similar, but weigh about 50% more. Weight comparisons will pick this up, though with the size of small gems the scale must be very sensitive.
• Diamonds are the hardest natural substance (even synthetic substances are only very slightly harder). If anything scratches a stone, it is not a diamond. If the gem can scratch any other object, it is probably genuine. (Cubic zirconiums are almost as hard as diamonds, so they might appear to pass this test. A cubic zirconium is 8 on the hardness scale, a diamond is 10).
• Most diamonds will give a blue fluorescence under ultraviolet light (UV), but a few high quality diamonds will not exhibit this property, so the test is an indication rather than conclusive proof.
• Diamonds do not appear on X-rays, where many other gems and solid substances do. This is one reliable test, though X-rays are expensive and only professionally done.