Crack open an unassuming stone, and you might just uncover a little conclave of crystals within. Their walls are glittering with quartz, amethyst, or calcite, protected by a hard shell that was shaped ...
Amethyst geodes—some stretching 5 m long—dot the ancient lava flows of Uruguay’s Los Catalanes mining area. The geodes contain calcite, colorless quartz, and agate, another quartz mineral. Large ...
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