gems, amblygonite

Gübelin describes transparent amblygonite as a potential gemstone, drawing attention to gem-quality material recognized during mineralogical research rather than through the discovery of a new deposit. In early 1953, G. Bottcher, a mineralogy student in Hamburg investigating the optical properties and orientation of spodumene, encountered pale-yellow crystals that A. Schroder identified as amblygonite. Although already well known as an economically important lithium mineral, amblygonite had received little attention as a faceted gem. Its composition, LiAl(OH,F)PO₄, permits variation through substitution between hydroxyl and fluorine, accompanied by changes in optical properties and specific gravity. The transparent material ranges from colorless to golden yellow, with pale yellow predominating, and resembles similarly colored spodumene. The crystals are triclinic and commonly have a slab-like habit. Gübelin distinguishes amblygonite’s single basal cleavage from the two cleavage directions of spodumene and reports that attractive stones have nevertheless been successfully faceted. With a Mohs hardness of 6, he considers the material suitable for wear. Measurements of the cut specimens are comparatively consistent, with average principal refractive indices of approximately 1.612, 1.621, and 1.636, birefringence near 0.026, and specific gravity between 3.01 and 3.03. The mineral is optically positive, with an optic axial angle of approximately 79°44′, compared with about 58° for spodumene. Microscopic examination reveals characteristic cloudy arrangements that may occupy most of a stone or form parallel and intersecting bands. At greater magnification, these clouds resolve into numerous minute cracks accompanied by particulate impurities resembling brown or black grains of sand; the particles are not mineralogically identified. Under long-wave ultraviolet radiation, the specimens exhibit faint, dirty greenish-yellow fluorescence. To verify the identification, Gübelin compares an X-ray powder diffraction pattern of the gem material with that of established amblygonite from Trujillo, Spain, recorded under identical conditions. Their agreement confirms that the transparent material is amblygonite. The article provides comparative physical measurements, photomicrographs of the inclusions, diffraction photographs, and a table of measured lattice spacings and relative line intensities. Determination of the lattice constants is deferred because of the triclinic structure and the difficulty of orienting an irregular fragment for rotation photography without a suitably developed crystal. Gübelin concludes that transparent amblygonite adds an interesting species to the range of gemstones, while judging that its probable scarcity will restrict its importance chiefly to collectors and gemologists rather than support a substantial public market.