San Carlos is the San Carlos Apache Reservation in southeastern Arizona, a tribal land covering approximately 7,500 square kilometres east of Phoenix. The reservation hosts the most productive peridot deposit in the United States and one of the more productive working peridot sources globally. Peridot occurs at Peridot Mesa, a Quaternary basalt flow within the reservation boundaries that contains olivine-rich xenoliths — fragments of the upper mantle brought to the surface by the volcanic activity. Mining is conducted under tribal management with rotating individual claims worked by tribal members, and the production has supplied the United States peridot market with consistent volume and quality since serious commercial activity began in the 1990s.

Geology

Peridot Mesa is a basalt flow of approximately one to two million years in age, part of the broader Cenozoic basaltic volcanism of the southwestern United States. Olivine xenoliths within the basalt range from a few millimetres to several centimetres across, with rare specimens larger. The xenoliths represent fragments of the lithospheric upper mantle that were entrained in the rising basalt magma during the eruption and brought to the surface relatively rapidly, before significant melting or alteration could destroy the xenolith material.

Most of the xenolith volume is dunite — nearly pure olivine — with subordinate clinopyroxene and minor accessories. The olivine crystallises in the orthorhombic crystal system, and the gem peridot extracted from the xenoliths is a forsterite-rich olivine with magnesium considerably exceeding iron in the typical Mg₂SiO₄–Fe₂SiO₄ solid solution. The forsterite-rich composition produces the characteristic yellowish-green to olive-green colour that is diagnostic of peridot and that distinguishes it from the more iron-rich olivines that occur in some other geological contexts.

Mining and production

Mining at San Carlos is principally surface and shallow-pit work, with tribal members working individual claims under licences managed by the San Carlos Apache Tribe. The xenoliths are recovered by breaking up the host basalt by hand and small mechanical methods, then sorting through the broken material to isolate the gem-quality crystals. The work is physically demanding and weather-dependent, and individual production volumes vary considerably with the diligence and luck of the working miners.

Total annual production from the reservation is estimated at several tonnes of rough peridot, with gem-quality cut stones representing a small fraction of the total weight. The bulk of cut stones falls in the range of one to ten carats, with stones over fifteen carats relatively unusual and stones over twenty-five carats rare. Larger stones occur but are typically the result of finding xenoliths larger than the typical few-centimetre size and are found infrequently.

The stones

San Carlos peridot ranges from yellowish-green through grass-green to olive-green, with the most desirable stones showing a clean medium-toned green with neither excessive yellow undertone nor excessive olive cast. The material is typically eye-clean to lightly included, with characteristic 'lily-pad' inclusions — disc-shaped strain features around small crystal inclusions — that are diagnostic of natural peridot. Heat treatment is not used; the colour is the natural product of the mineral chemistry and requires no enhancement.

Refractive indices around 1.654 to 1.690, specific gravity around 3.32 to 3.37, and hardness of 6.5 to 7 place peridot in the moderate range for jewellery use. The strong birefringence — about 0.036 — produces a characteristic 'doubling' of back facets visible through the table, which is a diagnostic feature in identification.

Position in the trade

San Carlos peridot supplies the bulk of the peridot sold in the United States market and a significant share of the global commercial peridot supply. The principal competition globally comes from Pakistan (Kohistan and Suppat), Myanmar (Mogok and Pyaung-Gaung), and historic Egyptian (Zabargad) production. San Carlos material is often slightly lower-priced than the finest Pakistani Kohistan stones but offers consistent supply and reliable quality at the commercial-stone size range. The deposit is the United States' most economically significant gemstone source and represents a notable example of indigenous-managed gemstone production at commercial scale.

In the trade

For working jewellers, San Carlos peridot is the default for moderate-priced peridot jewellery in the American market. The stones are reliably untreated, the supply is consistent, and the trade is established. Pricing for cut stones ranges from low double-digit dollars per carat for smaller commercial-grade material to mid-three-figure per-carat prices for fine stones over five carats; exceptional larger stones can command higher prices. Documentation of San Carlos origin is occasionally provided by independent laboratories on the basis of inclusion characteristics, though origin is rarely a determining factor in pricing at the commercial level.

Care

Peridot's hardness of 6.5 to 7 makes it acceptable for jewellery use but vulnerable to abrasion in daily-wear ring settings. The stone is sensitive to thermal shock and to acid exposure; ultrasonic and steam cleaning should be avoided, with mild soap and water the conservative cleaning protocol.

Further reading

• GIA — Peridot reference
• GIA Gems & Gemology — San Carlos peridot articles
• International Gem Society — peridot mineralogy and origins