Imitation gemstones merely imitate the color or look of a natural stone. They can be made out of anything. A cubic zirconia is an imitation diamond, for example. A blue piece of glass could be an imitation sapphire. A variety of blue synthetic forsterite is an excellent imitation for tanzanite, while chemically, it's composition is much closer to the yellowish green peridot. Another classic example is the "alexandrite" set in many jewelry pieces is actually a variety of vanadium colored synthetic sapphire.