A simulated gemstone is any stone which is being sold as something it is not. It can be made of a number of different materials, including glass, plastic, and, oddly enough, other synthetic or natural gemstones. For example, Alexandrites are often simulated by using lab-created color change sapphires. Natural zircon can be used to simulate diamonds or topaz. Simulated opals can be made of glass or plastic.
Synthetic spinels can simulate many stones, as they come in so many colors.
And many stones can be simulated by fusing layers of materials together into what are called doublets and triplets. Doublets and triplets can contain the actual gemstone as the top layer, fused to some other material which makes up the bulk of the stone. They require careful examination by a trained eye to detect. While most simulated stones are more commonly found in costume jewelry, some are out there passing themselves off as the real thing. However, it is fairly easy for a jeweler to tell the difference using common sense, magnification or gemological tests once he examines a stone carefully.