beryl, emerald, Austria, gems

Gübelin investigates whether emeralds from Habachtal, Austria, possess characteristics that distinguish them from emeralds of other origins, combining a review of mining history and geology with measurements and detailed microscopic examination. He recounts the deposit’s reported exploitation from antiquity onward and the repeated difficulties encountered by later mining ventures. Its position at approximately 2,100 metres in the Hohe Tauern, hazardous access, prolonged winters, modest production, and short mining licences are presented as obstacles to sustained commercial development. The emerald-bearing zone lies near the contact between central gneiss and surrounding schists. Gübelin interprets its formation through contact metamorphism and the interaction of granitic residual solutions with neighbouring rocks, assigning the beryllium supply to granite and the chromium responsible for green colour to serpentinite. Biotite schist is the principal host, accompanied by tremolite, actinolite, chlorite, and talc-rich rocks; he draws parallels with emerald deposits in the Urals, India, and the Transvaal. Most local beryl is pale or greyish green, while only a small proportion displays fine emerald colour, and abundant inclusions commonly diminish transparency and value. Crystals generally show simple prismatic forms, sometimes flattened according to their orientation within the schist. Measurements on eleven specimens establish average refractive indices of approximately 1.591 and 1.584, birefringence near 0.007, and an average specific gravity of 2.74. Dichroism is generally weak, combining yellowish green and bluish green, and the absorption spectrum is typical of emerald. Chelsea-filter reactions range from pink to red; ultraviolet fluorescence is usually suppressed, attributed to iron, although some specimens fluoresce red and the response under a Stokes fluoroscope is more conspicuous. The principal diagnostic evidence comes from inclusions and internal fractures. Gübelin describes frequent basal cracks, intersecting fracture systems, healed fissures, and liquid-filled cavities or negative crystals, commonly containing two phases and forming dense featherlike patterns. Biotite is the most abundant mineral inclusion, occurring as isolated plates, clustered books, or material extending inward from the matrix, sometimes altered or strongly resorbed. Other identified inclusions include tremolite, tourmaline, apatite, epidote, sphene, and rutile. Emeralds associated with tremolite-bearing rocks are generally clearer and less fractured than those from biotite schist, though often more bluish, and contain green needles, rods, or blades that may cross the entire crystal, terminate internally, form bundles, or bend. Gübelin regards the appearance of these tremolite inclusions as especially characteristic of Habachtal and valuable for recognizing cut stones, while acknowledging resemblance to amphibole inclusions in Ural emeralds. Photomicrographs document the inclusion assemblages, supporting the article’s central connection between geological environment, internal features, and the assessment of geographic origin.