Skarns can be found around the world and most developed during the Mesozoic period some 63 million to 230 million years ago. Zones of skarn are usually relatively irregular in shape due to the erratic characteristics of the host rocks, which vary in porosity, permeability, and lithology. The zones often contain deposits of iron, copper, gold, magnetite, tungsten, and other ores, though in relatively small amounts. Nevertheless, the small deposits are often mined for commercial profit. Some of the better-known ore-containing skarns can be found in Cornwall, Pennsylvania, Pine Creek, California, and the Copper Canyon of Nevada.