USA – Gem Origins Overview
US gem deposits – Montana sapphire, Utah red beryl, Arizona peridot, California and Maine tourmaline; geologically diverse patchwork provinces.
Introduction
The United States hosts geologically diverse gem deposits across unrelated provinces,
each reflecting a distinct geological setting: Montana sapphire in a lamprophyre dike
and alluvial placers, Utah red beryl in Eocene topaz rhyolite, Arizona peridot as
mantle xenocrysts in Quaternary alkali basalt, and California and Maine tourmaline
in LCT pegmatites.
Diagnostic significance is deposit-specific. Yogo Gulch sapphire is identifiable by
its low Cr/Ga ratio, low iron relative to Asian basaltic sources, natural untreated
blue, and flat tabular crystal habit. Utah red beryl is identified by Mn³⁺ as the
sole chromophore, fundamentally different from the Cr³⁺ of ruby or emerald, and
by bixbyite and topaz inclusions from its rhyolitic host. Arizona peridot from the
San Carlos Apache Reservation carries chromite inclusions with characteristic lily-pad
tension halos; San Carlos supplies an estimated 80–95% of world commercial peridot
by volume. [1]
No single fingerprint connects the US deposits to each other; each must be assessed
on its own gemmological criteria. [2]
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: [1]
- 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 [1] - 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 [2]
- Discrimination between California and Maine deposits, and between US and other
tourmaline origins, is a laboratory-level task requiring trace element chemistry
US Tourmaline and Arkansas Diamond – Notes
References
- ↑ 1. Koivula, J. (1981). San Carlos Peridot. Gems & Gemology, 17(4), 205–215. DOI: 10.5741/gems.17.4.205.
- ↑ 2. Schumann, W. (2009). Gemstones of the World (4th ed.). Sterling Publishing. ISBN: 978-1-4027-6829-3.