The cubic zirconia, an artificially made gem, is often used as a diamond stand-in. It resembles a diamond to the untrained eye and is much less expensive. With a little training, you can distinguish a real diamond from a cheap cubic zirconia without using high-tech equipment.
Look at the stone in bright light, such as in full sun, and notice its brilliance. A diamond should display only white or silvery sparkles that reflect more light. A cubic zirconia will throw off more fire, with colorful red, yellow and blue sparkles that look somewhat unnatural.
Consider the size of the stone. A large diamond of more than 2.5 carats is very rare and should cost a small fortune. Large cubic zirconia stones are common.
Examine the stone with a jeweler's loupe, and look at places where the facets of the stone meet. In a diamond, they will be sharp, but in a cubic zirconia they appear rounded.
Look deeply into the stone with a jeweler's loupe. A natural diamond should have some flaws in it. A synthetic cubic zirconia is perfectly flawless because it is made in the lab in controlled conditions. Flawless diamonds are not only rare, but extremely expensive.