Diamonds are some of the most precious, and expensive, stones on the planet. For many people, it is difficult to distinguish a real diamond from a manufactured stone, such as a cubic zirconium. Diamonds can be authenticated at a reputable jewellery store or by a gemologist, a person schooled in the art of working with gems and stones. This can also be very expensive, costing upwards of £195 an hour as of 2010. However, you can test a real diamond in the comfort of your home to make sure the price you paid was worth it.
Look at how the light reflects off the diamond in the sun. The diamond should have grey areas inside of it. If there is a rainbow reflection, either the diamond is of very low quality or it is a fake.
Check the weight of the stone by placing a cubic zirconium of the same size and shape onto a gram scale. Take this stone off the scale and place the stone in question onto it. The cubic zirconium should weigh about 55 per cent more than a real diamond. If the stones weigh approximately the same amount, the stone in question is probably a fake.
Breathe directly onto the stone. If the stone fogs for several seconds, it is a fake. The fog on a real diamond would fade almost immediately.
Look at the metals used to mount the stone, if there are any. Real diamonds are set in superior metals, such as 14 carat gold, 18 carat gold, platinum and silver. These metals are generally marked with symbols such as 14k, 18k, PT or 950. A cubic zirconium will probably be set in a gold- or silver-plated jewellery.
Look at the stone from the side. A real diamond will sparkle from this angle. A cubic zirconium when looked at from the side will appear dull and will have no sparkle.