Shigar Valley pegmatites

The list of what comes out of this valley is unusually long. A study by Agheem and colleagues in the Arabian Journal of Geosciences catalogued beryl in both its goshenite and aquamarine forms, tourmaline as schorl, garnet running between almandine and spessartine, plus apatite, topaz, fluorite, zoisite, clinozoisite and axinite. They ran the material through an electron probe microanalyser and an X-ray diffractometer and found each species chemically typical of its kind and homogeneous in composition, which is a quieter result than it sounds: it means the valley is producing textbook material, not oddities.

Where those minerals sit divides them into two groups. Beryl, tourmaline, garnet, apatite, topaz and fluorite occur in zoned pegmatites, grown by magmatic hydrothermal fluids into cavities and vugs in the intermediate zone. The zoisite, clinozoisite and axinite look different, and the authors suggest a metamorphic or metasomatic origin for those instead. Two processes, one valley.

Aquamarine is what carries the name internationally. Gems and Gemology reported in 2018 that northern Pakistan has become an increasingly important source, with a steady supply out of Gilgit-Baltistan, and Shigar named alongside the Hunza and Braldu valleys as the main producing areas. The crystals grow in zoned pegmatites, in cavities and veins opened by hydrothermal fluid. One Shigar aquamarine on feldspar, photographed for that report, stands seventeen centimetres tall.

That is the character of the district: a mineral common in the abstract, arriving here at a size that changes what it is for. The crystals grow as long hexagonal prisms reaching up to about a dozen centimetres, usually sitting on a matrix of albite, muscovite and schorl, and the report notes that most of them are collected as mineral specimens, with only parts of any given crystal gem quality and suitable for jewellery. The stone is worth more standing on its rock than it would be cut up.