Pollucite is a rare caesium aluminosilicate, ideally CsAlSi₂O₆, occasionally faceted as a colourless to white collector's gemstone. It belongs to the zeolite group structurally and forms in lithium- and caesium-rich granitic pegmatites in association with lepidolite, spodumene, and beryl. Facet-grade material is uncommon; gem use is limited; and the species sits firmly in the territory of the connoisseur collector and the mineralogical specialist rather than in the mainstream jewellery trade.

Composition and crystal habit

Pollucite forms a solid solution series with analcime, with caesium substituting for sodium across the series; the pure caesium end-member is uncommon, and most natural specimens carry a measurable analcime component. Crystals are isometric, typically forming as cubic or trapezohedral crystals, and the mineral is most often encountered as massive material rather than as well-formed crystals. The colour is usually colourless to white, occasionally with a faint pink, grey, or violet tint where impurities are present.

Hardness is 6.5 to 7 on the Mohs scale, with a refractive index of approximately 1.519 and a specific gravity of around 2.9. The relatively high specific gravity, driven by the heavy caesium content, is a useful diagnostic when distinguishing pollucite from analcime, leucite, or other isometric aluminosilicates. The lustre is vitreous on a polished surface, and cleavage is poor — a property that makes the material more workable for the lapidary than its zeolite-group affiliation might suggest.

Formation and sources

Pollucite is a late-stage mineral of highly evolved granitic pegmatites, where the residual liquids have concentrated the rare alkalis caesium, lithium, and rubidium to levels sufficient to crystallise dedicated mineral phases. The mineral occurs in association with petalite, spodumene, lepidolite, and beryl, and pegmatite bodies that contain pollucite are also typically the principal commercial sources of caesium for industrial use.

Notable gem-grade and specimen-grade pollucite localities include the Tanco mine at Bernic Lake, Manitoba, Canada — historically the world's largest single source of caesium and a producer of fine pollucite specimens; the Kunar and Nuristan pegmatite fields of Afghanistan, source of facet-grade transparent material that has reached the gem market in recent decades; the pegmatites of Maine and California in the United States; and pegmatites in Madagascar, Brazil, and Italy. The Afghan material, when available, has been the principal source of facet-grade pollucite for the international collector market.

Identification

Pollucite is identified by its isometric crystal system, its colourless to white colour, its hardness, its refractive index near 1.519, and most distinctively its specific gravity around 2.9, which separates it cleanly from analcime (around 2.27) and from leucite (around 2.5). Energy-dispersive X-ray fluorescence or other elemental analysis confirms the caesium content and is the definitive identification for laboratory work. Inclusions are typically small healed fractures or fluid inclusions characteristic of late-stage pegmatite minerals.

Faceting and gem use

Facet-grade pollucite is faceted in standard brilliant or step cuts, with cutters selecting designs that maximise the modest light return of a stone with a relatively low refractive index. The colourless body and absence of strong dispersion mean that pollucite, even when well-cut, presents a quiet, water-clear appearance rather than a fiery one. The material is used in jewellery rarely, primarily in collector pieces and in pegmatite-themed suites that combine pollucite with associated species such as petalite, spodumene, and morganite.

In the trade

Pollucite trades in the connoisseur and mineral-collector market rather than in the mainstream coloured-stone trade. Cut stones are most often available through specialist dealers in rare gems and through laboratory or museum-affiliated sources. Pricing reflects rarity rather than visual presence; a clean cut stone of two or three carats from Afghanistan or Tanco may command a meaningful premium relative to its visual appeal because of the species' scarcity and the limited supply of facet-grade rough.

Care

The hardness of 6.5 to 7 makes pollucite suitable for occasional-wear jewellery in protected settings, but the species is generally too soft for daily-wear ring use. Cleaning is by mild soap and warm water; ultrasonic cleaning is acceptable in the absence of fractures, but steam should be avoided.

Further reading

• GIA Gems & Gemology — rare gem species articles
• International Gem Society — pollucite reference
• GIA Gem Encyclopedia — rare and collector species