Quartz

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A colorless crystallized grouping of quartz. In good condition. The Rio Grande do Sul region in southernmost Brazil is a globally dominant source of amethyst quartz geode specimens, centered heavily around the municipality of Ametista do Sul. Geologically, the deposits are hosted within the massive basaltic lava flows of the Lower Cretaceous Parana Basin volcanic province. As these colossal flood basalts cooled, expanding gases and steam formed enormous, elongated vesicular cavities within the rock matrix. Late-stage, low-temperature hydrothermal fluids rich in silica and trace iron later circulated through the basalt fracture networks, slowly depositing bands of chalcedony followed by dense linings of well-formed, deep purple amethyst crystal prisms inside the hollow cavities. Artisanal surface prospecting and agricultural discovery of these geodes began in the late 1800s, with organized subterranean shaft and adit mining expanding dramatically during the mid-to-late twentieth century to supply international lapidary and collector markets. These underground mining cooperatives and private concessions have remained intensively active, modernizing their blasting and extraction techniques to yield a continuous supply of giant volcanic geode specimens well into the 2020s.