Dichroïsme is a characteristic of a uniaxial crystal showing a different colouring in the axis which is perpendicular to the optical axis..
Cleavage is a plane according to which a crystal can easily split.
A doublet is a gluing joint of two substances to imitate a gem (if more than two substances are used, the result is called triplet or quartet).
As for counterfeiting, any colour collection gem of a certain value should be examined by a certified gemology laboratory.
Anyone willing to improve his knowledge of gems should train his eye observation using a 10x magnifying glass.