Anhydrite is a calcium sulphate mineral with the chemical formula CaSO₄, belonging to the orthorhombic crystal system. It is the anhydrous counterpart of gypsum (CaSO₄·2H₂O), differing from that more familiar mineral solely in the absence of structurally bound water molecules. Although anhydrite is widespread in geological terms — occurring in thick evaporite sequences on every inhabited continent — its use as a gemstone is narrow and specialist, constrained by a Mohs hardness of only 3 to 3.5 and three directions of perfect cleavage that render it fragile and poorly suited to everyday wear. Its principal gem interest lies in a blue to blue-grey variety marketed under the trade name angelite, sourced primarily from Peru, which has achieved modest but genuine commercial traction since the late twentieth century. Collectors, lapidaries, and metaphysical-mineral enthusiasts constitute the core market; fine jewellery applications remain rare and require protective settings.

Mineralogy and Crystal Chemistry

Anhydrite crystallises in the orthorhombic system, space group Amma, with sulphate tetrahedra linked by calcium ions in eight-fold coordination. This architecture produces a dense, compact structure — denser, in fact, than the hydrated gypsum into which it readily converts under surface conditions. The theoretical specific gravity of pure anhydrite is approximately 2.89 to 2.98, noticeably higher than gypsum's 2.31 to 2.33, a distinction useful in identification. The refractive indices are low and biaxial: α ≈ 1.569, β ≈ 1.574, γ ≈ 1.614, giving a birefringence of approximately 0.044. The optical character is biaxial positive. Dispersion is negligible, so anhydrite displays no fire; its appeal is entirely chromatic and textural.

The three cleavage planes — perfect on {010}, good on {100}, and good on {001} — intersect at or near right angles, producing a characteristic blocky or rectangular parting that is immediately apparent when a specimen is fractured. This cleavage geometry, combined with the low hardness, makes faceting anhydrite an exercise in patience rather than practicality; the vast majority of gem-quality material is fashioned into cabochons, beads, tumbled stones, or carved objects.

Anhydrite is stable under dry, high-pressure conditions but is metastable at the Earth's surface, where it slowly hydrates to gypsum. This conversion, which involves a volume increase of roughly 60 per cent, can cause structural disruption in evaporite sequences and is of considerable importance in civil engineering, though it proceeds slowly enough that gem specimens remain stable under normal storage conditions.

Colour and Optical Character

Pure anhydrite is colourless to white. Gem-quality material occurs in several colour varieties:

• Blue to blue-grey (angelite): The most commercially significant variety, ranging from pale sky blue through medium blue-grey to a distinctive lilac-blue. The colouration is attributed to the scattering of light by fine inclusions and structural irregularities rather than to a chromophoric ion in the strict sense, giving the material a slightly milky, translucent character rather than the transparency of a facetable gem.
• Violet to lavender: Occasionally encountered, sometimes overlapping with the angelite colour range. Certain specimens from German and Austrian evaporite deposits display a soft lavender hue.
• Colourless to white: Massive or granular colourless anhydrite is common but of negligible gem interest.
• Reddish or pinkish: Rare; caused by included iron oxides or haematite dust along cleavage planes.

The lustre of polished anhydrite is vitreous to pearly, the pearly effect being particularly pronounced on cleavage surfaces. Transparency ranges from translucent to nearly opaque in the blue varieties, which typically show a waxy to greasy surface lustre when cabochon-polished.

Geological Occurrence and Formation

Anhydrite forms principally in marine evaporite sequences, precipitating from concentrated brines after the removal of carbonates and halite. It is a characteristic constituent of the evaporite succession — alongside halite, gypsum, sylvite, and dolomite — and occurs in beds that may reach tens of metres in thickness. Major evaporite basins containing anhydrite include the Permian Zechstein Basin of northern Europe, the Triassic evaporites of the Alpine foreland, and numerous Mesozoic and Tertiary basins in the Americas and the Middle East.

Anhydrite also forms as a hydrothermal mineral in veins and as a contact-metamorphic product, though these occurrences are less likely to yield gem-quality material. At depth, where temperatures exceed approximately 40 °C and pressures are elevated, anhydrite is the stable phase relative to gypsum; at shallower levels, the reverse is true, and anhydrite converts progressively to gypsum.

The blue angelite variety is associated with a specific geological context in Peru: massive nodular or bedded anhydrite in evaporite-related sequences in the Peruvian Andes, where the blue-grey colouration develops through a combination of trace impurities and the fine-grained, microcrystalline texture of the rock. The material is typically mined as nodules or slabs, then cut and polished locally before export.

Principal Localities

Gem-quality anhydrite localities of commercial or mineralogical significance include:

• Peru (Ica and Ancash regions): The primary source of blue angelite, discovered in commercial quantities in the 1980s. Peruvian material dominates the international trade and sets the benchmark for colour and quality in this variety.
• Germany (Stassfurt and Hannover evaporite basin): Historically important for fine crystallised anhydrite specimens, including colourless and pale violet crystals of collector quality. The Stassfurt potash mines yielded notable cabinet specimens in the nineteenth and early twentieth centuries.
• Austria (Hallstatt and Berchtesgaden salt districts): Alpine evaporites have produced lavender and blue-grey anhydrite, occasionally of gem interest.
• Mexico (Naica, Chihuahua): The Naica mine, celebrated for its giant selenite (gypsum) crystals, also yields anhydrite in associated veins, though gem-quality blue material is not the primary product here.
• United States (New Mexico, Texas, Kansas): Evaporite deposits in the Permian Basin yield anhydrite, primarily of industrial rather than gem significance.
• Libya and Egypt: North African evaporite sequences contain anhydrite, occasionally with attractive colouration, though these sources are not systematically exploited for the gem trade.

Angelite: The Principal Gem Variety

Angelite is a trade name, not a mineralogical designation, applied to the blue to blue-grey massive anhydrite from Peru. The name entered the gem and mineral trade in the late 1980s, coinciding with the opening of Peruvian deposits and the growth of the metaphysical-mineral market. It has since become sufficiently established that it appears in trade catalogues, bead suppliers' inventories, and lapidary publications as a recognised commercial entity, though it carries no formal standing in mineralogical nomenclature.

The characteristic colour of angelite — a soft, slightly milky blue reminiscent of pale aquamarine or blue lace agate — derives from the microcrystalline texture of the material and the way it scatters incident light. The effect is analogous to the diaphanous blue of certain chalcedonies, though the mechanism and mineralogy are entirely different. High-quality angelite is uniformly coloured, free of brown or grey patches, and takes a smooth, even polish. White or colourless patches and brownish staining along cleavage planes are common in lower-grade material.

Angelite is fashioned almost exclusively into cabochons, beads (round, oval, and rondelle), tumbled stones, and small carvings. Faceted stones are technically possible but rarely attempted commercially, as the cleavage makes faceting hazardous and the translucency limits the optical return. Cabochon-cut angelite is used in silver jewellery — pendants, earrings, and brooches — where the stone is protected by a bezel setting. Ring use is inadvisable given the hardness and cleavage.

Gemmological Identification

Distinguishing anhydrite from superficially similar blue or blue-grey gem materials is straightforward for a trained gemmologist:

• Hardness: At 3 to 3.5, anhydrite is scratched easily by a copper coin (hardness ~3) or a steel blade. This alone separates it from blue chalcedony (hardness 6.5–7), blue lace agate, and aquamarine.
• Specific gravity: The SG of approximately 2.89 to 2.98 is higher than that of blue chalcedony (~2.60) and distinguishable from blue calcite (~2.71).
• Refractive index: The RI range of approximately 1.569 to 1.614 is measurable on a refractometer for polished flat surfaces, though the birefringence may cause a blurred shadow edge on cabochons.
• Cleavage: The three near-orthogonal cleavage directions are visible under magnification as parallel sets of fine fractures or surface steps, a characteristic feature.
• Reaction to water: Anhydrite does not effervesce with dilute hydrochloric acid (distinguishing it from blue calcite and aragonite), and it does not dissolve rapidly in water under normal conditions.
• Fluorescence: Anhydrite may show weak to moderate orange or yellowish fluorescence under long-wave ultraviolet light, though this is variable and not diagnostic.

The most common confusion in the trade is between angelite and blue calcite, which shares a similar colour range and translucency. Blue calcite is softer (hardness 3), lower in SG (~2.71), and effervesces with acid — a definitive test. Howlite (hardness 3.5, white with grey veining, sometimes dyed blue) and blue lace agate are also occasionally confused with angelite by non-specialists.

Treatments and Enhancements

Anhydrite and angelite are not routinely treated in the manner of corundum or beryl. The principal concern in the trade is dyeing: white or pale anhydrite can be dyed blue or other colours to simulate higher-grade angelite or other gem materials. Dyed material may be detected by concentrated colour along surface-reaching cleavage planes and fractures, uneven colour distribution under magnification, and the transfer of colour to an acetone-dampened cotton swab in some cases.

Impregnation with colourless resins or waxes to improve surface stability and polish is possible in principle, as it is for other porous or soft minerals, though it is not documented as a widespread practice in the angelite trade. Buyers of angelite at the wholesale level should be aware that the material's porosity makes it susceptible to absorption of oils, cosmetics, and perspiration over time, which can alter the surface colour.

Care, Durability, and Practical Considerations

Anhydrite's combination of low hardness and perfect cleavage in three directions places it firmly in the category of collector and display stones rather than everyday jewellery gems. The following practical points apply:

• Anhydrite will scratch from contact with most other gem materials, metals, and even fingernails under pressure. It should be stored separately, wrapped in soft cloth.
• Prolonged exposure to water or high humidity should be avoided, as anhydrite is metastable with respect to gypsum and surface hydration, though this conversion is slow under ambient conditions.
• Ultrasonic and steam cleaners are contraindicated; cleaning should be limited to a soft, dry or barely damp cloth.
• Acid cleaning agents, including common household cleaners, will etch the surface.
• Jewellery incorporating angelite is best limited to pendants, earrings, and brooches in protective bezel settings. Rings and bracelets expose the stone to unacceptable abrasion risk.

Market Context and Trade

Anhydrite occupies a modest but stable niche in the coloured-stone trade. The angelite variety from Peru is the dominant commercial form, sold primarily through bead suppliers, lapidary wholesalers, and metaphysical-mineral retailers. Prices are low relative to most gem-quality minerals: cabochons and beads are traded at commodity-level prices per strand or per kilogram, reflecting the abundance of the material and its limited durability. Fine individual cabochons with excellent colour and polish command a premium within the category but remain inexpensive by the standards of the broader gem market.

Crystallised anhydrite specimens — particularly well-formed, transparent to translucent crystals from European evaporite localities — are collected by mineral specimen enthusiasts and may command prices reflecting their aesthetic and scientific interest rather than their gem utility. The Stassfurt and Austrian material, though no longer actively mined for specimens, appears regularly in mineral shows and auction catalogues.

The metaphysical-mineral market has been the primary driver of angelite's commercial success since the 1990s, and the stone's trade name, with its connotations of celestial blue and spiritual association, was clearly chosen with this market in mind. Gemmologists and jewellers should be aware that the name angelite carries no mineralogical authority and that the material is, in all scientific respects, massive blue anhydrite.

Further Reading

• GIA Gem Encyclopedia — gia.edu
• International Gem Society: Anhydrite Gem Information — gemsociety.org