USA – Gem Origins Overview
US gem deposits – Montana sapphire, Utah red beryl, Arizona peridot, California and Maine tourmaline; geologically diverse patchwork provinces.
By gemmology.dev editors
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usa montana utah arizona california sapphire red-beryl peridot origin/usa
Introduction
The United States hosts geologically diverse gem deposits with no single dominant
orogenic province. Major deposits include: Montana sapphire (lamprophyre and
alluvial), Utah red beryl (rhyolitic volcanic), Arizona peridot (basaltic xenocyrst),
and California/Maine tourmaline (LCT pegmatite). Each deposit reflects a distinct
geological setting; no common chemical or inclusion fingerprint links them.
Major US Gem Deposits
| State | Deposit | Host Rock | Gems |
|---|---|---|---|
| Montana | Yogo Gulch, Rock Creek, Missouri River | Lamprophyre dike + alluvial placers | Sapphire |
| Utah | Wah Wah Mountains | Topaz rhyolite (Eocene) | Red beryl |
| Arizona | San Carlos Apache Reservation | Quaternary alkali basalt (xenocryst) | Peridot |
| California | Pala District, San Diego County | Cretaceous LCT pegmatite | Elbaite tourmaline |
| Maine | Oxford County pegmatites | LCT granite pegmatite | Elbaite tourmaline (blue/green) |
Montana Sapphire – Brief Overview
- Three deposit types: Yogo Gulch (lamprophyre), Rock Creek (alluvial), Missouri River
- Yogo: steely blue, rarely >2 ct, no heat treatment needed; low Fe; lamprophyre host
- Rock Creek / Missouri River: pastel multicolour; mostly heat-treated for commercial blue
- See dedicated file: origin/usa/montana-sapphire
Utah Red Beryl – Brief Overview
- Wah Wah Mountains, Beaver County – the world's only commercial red beryl source
- Hosted in Eocene topaz rhyolite; Mn³⁺ chromophore; typically <0.5 ct
- See dedicated file: origin/usa/utah-red-beryl
Arizona Peridot – San Carlos
San Carlos Apache Reservation (Gila County) produces an estimated 80–95% of
commercial peridot by volume:
- Host: Quaternary alkali basalt; peridot is a mantle xenocryst
- Composition: Mg₂SiO₄–Fe₂SiO₄ (olivine); San Carlos material ~Fo₈₆–Fo₉₂
- Properties: RI 1.654–1.712 (α–γ); biaxial positive; birefringence 0.035–0.038
(strong facet doubling at 10×); SG 3.28–3.48; Hardness 6.5–7 - Colour: Yellow-green to lime-green (Fe²⁺ chromophore)
- Absorption: Three-banded iron spectrum: 493, 473, 453 nm (Foundation examination requirement)
- Key inclusions: Chromite (spinel) with lily-pad tension fracture halos (Diploma
requirement); ludwigite platelets; biotite; fluid inclusions - Origin discrimination: Not practical at bench level; San Carlos vs Hawaiian or
Chinese peridot requires quantitative microprobe analysis
US Tourmaline (Brief)
California (Pala District) and Maine (Oxford County) tourmaline:
- Both produce elbaite (Na(Li,Al)₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₄) from LCT-type pegmatites
- Colour range: pink/red rubellite, blue indicolite, bicolour, watermelon, parti
- RI: 1.614–1.679 (uniaxial negative); SG: 3.01–3.06; birefringence: 0.014–0.021
- California: historical Qing Dynasty export; Maine: first commercial North American tourmaline
- Discrimination between California and Maine deposits, and between US and other
tourmaline origins, is a laboratory-level task requiring trace element chemistry