Fordite (also called Detroit agate or motor agate) is made up of layers of hardened automobile paint from the oversprayed paint when people used to hand spray paint onto cars as part of the manufacturing process. When the paint got hard enough, this enamel material could be cut and polished. These “stones” have a variety of colorful designs and are often cut into cabochons to be used in jewelry.
Rainbow calsilica’s history and origin is less clear. Like fordite, rainbow calsilica is known for having a variety of colors and patterns. One supplier claims the material comes from mines in Mexico. The Gemological Institute of America lab tested the material and states it is man-made. Another lab, SSEF in Basel, Switzerland came to the same concluion. While lovely, rainbow calsilica “[c]onsists of dyed sand that has been firmed with artificial resin.” 
 Gemstones of the World: Newly Revised & Expanded Fourth Edition by Walter Schumann, p282