Chert is very common in alluvial gravels because the stuff is hard, tough, and insoluble. It survives conditions of water transport that tend to disintegrate most other rocks. Color is determined by the impurities. Reds and yellows are usually ferric iron, grays and dull blues and dull greens are usually ferrous iron. Chert that forms in ore minerals is often much brighter, being colored by other elements such as copper. Brightly colored cherts are categorized as semiprecious stones. Certain varieties are fairly rare and valuable, others like jasper are common and inexpensive but nonetheless good stuff for making inexpensive jewelry and works of art.