Augelite is a rare aluminium phosphate hydroxide mineral with the chemical formula Al₂(PO₄)(OH)₃, belonging to the orthorhombic crystal system. It forms colourless to pale yellow — occasionally pale green — transparent crystals that, when of sufficient clarity and size, are occasionally faceted as collector's gemstones. Its combination of low hardness, scarcity of gem-quality rough, and restricted localities ensures that faceted augelite remains firmly within the province of the specialist mineral and gemstone collector rather than the jewellery trade. Nonetheless, well-cut specimens command genuine interest at mineral shows and in specialist auctions, precisely because clean, facetable material is so seldom encountered.

Physical and Optical Properties

Augelite crystallises in the orthorhombic system, typically producing tabular or prismatic crystals with a vitreous to pearly lustre on cleavage surfaces. Its hardness on the Mohs scale falls between 4.5 and 5 — comparable to apatite — which renders it susceptible to scratching and abrasion, and effectively rules out use in rings or bracelets. The specific gravity is approximately 2.70, placing it in the lighter range among phosphate minerals.

Optically, augelite is biaxial positive, with refractive indices of approximately α = 1.574, β = 1.576, and γ = 1.588, yielding a birefringence of roughly 0.014. The 2V angle is relatively large, typically reported in the range of 50–54°. These optical constants, while not dramatically distinctive, allow a trained gemmologist using a refractometer and polariscope to distinguish augelite from superficially similar colourless phosphates and other low-index minerals. Dispersion is low, so faceted stones display little fire; their appeal lies instead in their water-clear transparency and the intellectual satisfaction of owning a correctly identified rarity.

• Crystal system: Orthorhombic
• Chemical formula: Al₂(PO₄)(OH)₃
• Hardness (Mohs): 4.5–5
• Specific gravity: ~2.70
• Refractive indices: ~1.574–1.588
• Birefringence: ~0.014
• Optical character: Biaxial positive
• Lustre: Vitreous; pearly on cleavage
• Cleavage: Perfect in one direction, good in another
• Colour: Colourless, pale yellow, rarely pale green

Geological Occurrence

Augelite forms as a secondary or late-stage hydrothermal mineral in phosphate-rich metamorphic and pegmatitic environments. It is most commonly associated with other phosphate minerals — including wavellite, lazulite, and variscite — as well as quartz and various aluminium-bearing silicates. The mineral was first described in 1868 by the Swedish mineralogist Nils Adolf Erik Nordenskiöld, who named it from the Greek augē, meaning lustre or brightness, in reference to the mineral's distinctive pearly cleavage sheen.

The two localities most consistently cited as sources of gem-quality or near-gem-quality material are White Mountain in Mono County, California, USA, and various phosphate deposits in Bolivia, particularly in the Potosí department. The Californian material occurs in a metamorphic context associated with lazulite-bearing quartzites, while Bolivian specimens are associated with tin-bearing hydrothermal veins and phosphate-rich alteration zones. Sweden, where the mineral was first characterised, has also yielded mineralogically important specimens, though rarely of facetable quality. Additional occurrences have been recorded in Brazil and Australia, but gem-grade material from these localities is not well documented in the gemmological literature.

Gem-Quality Material and Faceting

Facetable augelite is genuinely scarce. Clean crystals of sufficient size — typically above five millimetres in their largest dimension — to yield a presentable cut stone are rarely encountered even at specialist mineral shows. When such material does appear, it is most commonly fashioned into small brilliant or step-cut stones, generally under two carats, intended purely for collection. The perfect cleavage in one direction presents a meaningful challenge to the lapidary, demanding careful orientation of the rough to minimise the risk of cleavage during cutting and polishing.

Finished stones are colourless and glassy in appearance, with a refractive index low enough that they lack the brilliance of higher-index gems such as topaz or sapphire. The appeal is therefore entirely one of rarity and scientific interest: a correctly identified, well-cut augelite represents a genuine achievement in both lapidary skill and gemmological identification. Weights above one carat are uncommon; stones above two carats are exceptional and would be noteworthy in any specialist collection.

Identification and Separation from Similar Minerals

Because augelite is colourless and of relatively low refractive index, it could theoretically be confused with a range of other colourless minerals including quartz, topaz (at lower RI readings if measurement error is introduced), apatite, and other phosphates. In practice, the combination of its specific refractive indices, biaxial positive optical character, relatively low specific gravity, and moderate hardness provides a reliable diagnostic suite when a full set of gemmological instruments is employed. The pearly lustre visible on natural cleavage surfaces is a useful field indicator. Raman spectroscopy and X-ray diffraction provide definitive identification where instrument access is available, and any serious collector acquiring an augelite of significant size would be well advised to obtain laboratory confirmation.

In the Collector's Market

Augelite occupies a small but stable niche within the world of rare faceted collector gemstones — a category that encompasses minerals prized not for wearability but for their mineralogical significance, crystallographic interest, and the challenge they represent to the lapidary. Fine mineral specimens of augelite, particularly well-formed Bolivian crystals with good transparency, appear periodically at major mineral shows such as the Tucson Gem and Mineral Show and at specialist European mineral fairs. Faceted stones are rarer still and tend to circulate among a small community of collectors who maintain comprehensive suites of unusual phosphates or rare orthorhombic minerals.

There are no known treatments applied to augelite; the material is too soft and too rarely encountered in gem quality for any enhancement programme to be commercially meaningful. What the collector acquires is, in every case, entirely natural and unmodified rough that has been shaped by a skilled lapidary. This unimpeachable naturalness, combined with genuine rarity, gives augelite a quiet but enduring appeal within its specialist audience.

Further Reading

• Gems & Gemology — GIA Publications
• International Gem Society — Rare Gemstones Overview