When it first became commercially available many years ago, cubic zirconia (or "CZ") fooled quite a few people. It doesn't anymore, and many can tell it's not a real diamond at arm's length. Nonetheless, there are some good fakes out there, and you need to be careful if dealing with a seller you don't know well.

Very recently introduced in the marketplace, moissanite, a form of silicon carbide, is considered by many to be an even better simulant than CZ. Its physical and optical properties are much closer to a real diamonds. Qualified gemologists and jewelry experts can tell the difference between diamond and moissanite. Less experienced jewelers may have difficulty separating Moissanite from diamond, as Moissanite reacts like diamond with traditional thermal testing devices. Opportunities for fraudulent misrepresentation will, no doubt, abound with this new material.