Tanzanite is a relative newcomer to the jewelry scene, having been around only since the 1960s. It was first brought to public attention by Tiffany & Co. and was originally prized for its similarity to high-end Ceylon sapphires but at a much lower cost. Tanzanite, however, has become desired on its own for its dramatic rich bluish-violet color. It is found only in one place in the world, the African nation of Tanzania, for which the stone was named. Since the supply is fairly well-controlled, there is not as much of a threat or opportunity for fakes, but imitations do find their way into the market. There are a few ways to detect real tanzanite from fake.
Look at the stone in both natural and incandescent light from a bulb. True tanzanite is trichroic and displays three colors. The stone should show blue with a tinge of violet in natural light, but should display flashes of red and pink when moved around in incandescent light.
Look through the side of the stone. It should show the same intensity of color from the side as it does from the top. Imitations like iolite do not; they will appear clear or even yellowish from angles other than the top.
Examine the stone thorough a 10X jeweler's loupe. This is required because tanzanite does not typically have flaws that can be seen with the naked eye. If the stone has no flaws in it when viewed with the loupe, there is a good chance that it is not a tanzanite. Imitation or lab-created stones often have fewer flaws (or no flaws) than natural gemstones.
Look for good refraction, or depth of sparkle, in the stone. Colored glass, which is sometimes used as a simulant, will look a little "flat."