Quartz

specimen number:
2292356

location:
Rio Grande do Sul, Brazil

description:
A large basically geodal tube of agate, the interior lined with purple amethyst crystals. A small hole does run the length of the tube. Crystallized on both sides as shown in the video.

The amethyst deposits of Rio Grande do Sul, Brazil, are situated within the Serra Geral Formation, part of the Paraná Continental Flood Basalt Province formed by massive volcanic eruptions during the Early Cretaceous period, roughly 135 million years ago. As the extensive basaltic lava flows cooled, escaping gases and hydrothermal activity created large, hollow cavities, or geodes, within the rock. Over time, low-temperature, silica-rich groundwater heated by geothermal gradients circulated through these cavities, gradually depositing layers of chalcedony, colorless quartz, and finally amethyst. The purple coloration of the crystals developed due to trace amounts of iron incorporated into the quartz structure, which was later exposed to natural ionizing radiation from the surrounding basalt. Historically, European immigrants discovered these deposits in the early nineteenth century, and subsequent mining in districts like Ametista do Sul transitioned from surface gathering to extensive underground galleries, establishing the region as a primary global exporter of basalt-hosted amethyst geodes.

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