Smoky quartz ranges from a brownish gray to black. The brownish gray crystals are transparent, while darker ones are opaque. Smoky quartz forms from natural gamma radiation released by surrounding rocks. The radiation causes aluminum atoms, which replace silicon atoms in the crystal lattice, to transfer an electron to a neighboring positively charged ion, often Li+, forming an aluminum metal color center that darkens the quartz. These specimens are black and opaque.
Mined in Minas Gerais, Brazil, with no precise location given. The pencil is 5 1/2"long, for scale. Good termination at one end, with most having the striations perpendicular to the length of the crystal that are characteristic of quartz.
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