Just like jewelry diamonds, alexandrite has many stimulants. There is a simulated variety on the market, which is made out of corundum instead of chrysoberyl, containing tiny amounts of vanadium to mimic the color change seen in real alexandrite. Synthetic sapphires sometimes get passed off as alexandrite, carrying a range of changing color from mauve to bright purple, inside a single stone. You can have the stone tested in a lab to see what it is actually made of. The easiest check is that of the refractive index – chrysoberyl has this at 1.741-1.760, while corundum has a refractive index of 1.759-1.778.
There are also color garnets that you could confuse for alexandrite, as well as rough andalusite. These are not imitations; rather they are lo\wer-priced yet genuine gemstones. Always make sure you get what you pay for.