You can! The results would be super Mojave (BEL), super Special, or more crystals. No normals.
You just can't get another crystal if you breed it to something that does not have Mojave or special.
For example:
Crystal x Normal = Mojaves or specials
Crystal x Mojave= Mojaves, specials or crystals
Crystal x Enchi = Enchi special, Enchi Mojaves, Mojaves or specials