Multi-colored Calsilica some times called Rainbow Calsilica has an interesting history as its origin have been discussed, since it was first introduced into the jewelry trade, as to it being natural or man made. The legend has it being mined in Mexico, and that's when the debate started as to its real origins. The paragraph above shows it as a man made material but one that still has a lot of appeal to consumers  and collectors. 

"Rainbow Calsilica is an artificial material created for carving and lapidary purposes. It contains a mixture of calcite, plastic and artificial coloring pigments (PB15, an art-supply pigment), and a paraffin-like material. [ref: Winter 2002, Gems and Gemology, journal of Gemological Institute of America]. 

Although suppliers claim it to be a natural stone, tests have indicated that it is probably powdered carbonate rock artificially colored and then stabilized with plastic. Never the less it is still very, very ....BEAUTIFUL.