thanks for the additional info:
crystals are limited only to your imagination. Carving Wax will give you very sharp edges. Super Sculpey takes a bit more "handling" to get the shapes right, but can be done. Of course there is wood, if you are a woodworker and have the various hand and power tools to make the shapes.
Whatever modeling clay you use, it must be "sulphur free". A lot of rubber molding materials will not cure when in contact with sulphur or latex.
what will the project be used for once it is molded and cast ?
are you going to make just one cast or several ?
here is just "one" example of making faux-crystals: