Gemstone deposits of the Four Corners region, USA
— Robert W. Eveleth and Virgil W. Lueth

Collecting of gem olivine (peridot), pyrope garnet, and chrome diopside has occurred in the four corners region prior to the arrival of Europeans and probably long before. These gemstones are derived from lower crustal and mantle xenoliths entrained in the volcanic necks and tuffs of the Navajo Volcanic Field. Subsequent weathering has liberated these minerals from their matrices leaving residual deposits of gemstones. Another concentration mechanism for these gems is reworking by ants and subsequent accumulations on their hills. Although these stones have been gathered, traded, or sold around the southwest for over 500 hundred years, perhaps longer, commerical mining is not likely given cultural prohibitions and low volume of commerical material.