Amethyst

Amethyst forms in volcanic rock cavities called geodes. When silica-rich groundwater seeps through volcanic rock, it slowly deposits layers of quartz crystals over millions of years. The purple color comes from trace amounts of iron (Fe³⁺) and natural irradiation within the Earth's crust. The intensity of the color depends on the concentration of iron and the amount of radiation the crystal receives during formation.

Cathedral geodes from Uruguay can reach over two meters tall, their interiors lined with deep violet crystals that took tens of millions of years to grow. In Brazil's Rio Grande do Sul, entire basalt flows contain hollow cavities that became crystal-lined chambers — some large enough to stand inside.

Amethyst is relatively easy to identify by its distinctive purple color and hexagonal crystal habit. It has a Mohs hardness of 7, meaning it cannot be scratched by a steel knife. The color ranges from pale lavender to deep royal purple, often with color zoning visible as angular bands within the crystal.

Unlike purple fluorite (which it's sometimes confused with), amethyst has no cleavage and breaks with a conchoidal fracture. Under UV light, some amethyst specimens will fluoresce a weak blue-green. It's also significantly harder than fluorite (7 vs 4 on the Mohs scale).