Peridot Mesa — The San Carlos Apache Reservation as the World's Volume Source for Peridot
Peridot Mesa — The San Carlos Apache Reservation as the World's Volume Source for Peridot
An Arizona basalt-hosted deposit producing the bulk of the world's commercial-grade peridot under tribal lease
Peridot Mesa is the trade name for the peridot-producing basalt deposits on the San Carlos Apache Reservation in Arizona, a tribal nation of approximately 7,200 square kilometres in the eastern part of the state. The mesa accounts, by most credible industry estimates, for 80 to 90 percent of world peridot production by tonnage, supplying the volume commercial-jewellery market with material in the under-three-carat range that comprises the bulk of August birthstone retail. Mining is conducted by enrolled members of the San Carlos Apache Tribe under lease and concession arrangements administered by the tribal government, and the production is one of the more economically consequential commercial gem operations under direct tribal control in North America.
Geology
Peridot Mesa is a Pliocene-age basalt flow that erupted onto the surrounding terrain approximately three million years ago. The basalt is an alkali olivine basalt with abundant olivine xenocrysts — fragments of upper-mantle peridotite plucked from the depth at which the magma originated and carried to the surface in the rapidly ascending volcanic conduit. The olivine xenocrysts are the peridot. Weathering of the basalt over the intervening million years has released the harder olivine crystals from the softer basalt matrix, concentrating them in the surface soils and stream gravels of the mesa.
Mining therefore consists of working the weathered surface layer of the basalt and the secondary alluvial concentrations rather than the deeper unweathered rock. The technique is a combination of pick-and-shovel work, washing and screening, and hand-sorting of the recovered olivine. Modern operations have introduced mechanical screening and trommels, but the work remains largely manual.
The peridot
San Carlos peridot is typically a yellowish-green to slightly olive-green colour, with the iron content of the olivine producing a bodycolour that is generally lighter and more yellow-tinted than the deeper grass-green of the Pakistani Sapat material. Crystal sizes are modest: clean facetable rough above three carats is uncommon, and stones above five carats clean and well-coloured are rare in San Carlos production. The bulk of the cut goods are stones from approximately 0.25 to 2 carats, sized to suit accent stones and modest centre stones in commercial jewellery.
Inclusions are common and include the diagnostic lily pad stress halos around small chromite or other dark mineral inclusions characteristic of olivine. Eye-clean material is the commercial standard, but heavily included rough is also worked into less-expensive cut goods.
The economic context
Peridot mining has been one of the principal income sources for the San Carlos Apache Reservation for several decades, providing employment for tribal members in mining, sorting, cutting (some material is cut on or near the reservation), and trading. The tribal government administers leases and concessions and collects royalties on the production. The market for San Carlos peridot is principally the North American commercial jewellery trade, with smaller volumes moving into international markets where Pakistani and Burmese material commands the upper-tier positions.
In the trade
Buyers of San Carlos peridot should expect material at modest per-carat prices in sizes typical of accent and commercial centre stones. The colour difference between San Carlos and Pakistani peridot is visible to the trained eye and is the principal reason Pakistani material commands a substantial premium for any given size. For mass-market August birthstone work, San Carlos production is the natural default; for upper-tier custom jewellery and for centre stones above five carats, the Pakistani Sapat market is the more usual source.