Smoky quartz ranges from a brownish gray to black. The lighter 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 points were mined from cavities in a pegmatite near Diamantina in Minas Gerais, southeastern Brazil.
Useful teaching specimen with perfect termination at one end. Click large image for a larger view.
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