Two hundred meters below the basalt plains of Rio Grande do Sul, the rock opens into a cavity no wider than a dining room. The walls are lined with crystals — thousands of hexagonal prisms catching the beam of my headlamp and scattering it into violet fragments. The largest specimens reach nearly a meter in length, their tips clouded with a milky quartz that diffuses light the way frosted glass does.
How a Geode Begins
The formation begins with violence. Roughly 130 million years ago, lava flows from the Serra Geral formation pushed across the Paraná Basin, filling valleys and hardening into basalt. Gas pockets trapped within the cooling rock left hollow cavities that would become, over incomprehensible spans of time, mineral rooms lined with amethyst.