What is a key locality known for hydrothermal gems?

Ouro Preto, Brazil, is renowned for its significant hydrothermal gem deposits, particularly for the production of quartz varieties, as well as other gemstones like tourmaline and topaz. The geological conditions in this region created a favorable environment for the formation of these gems through the process of hydrothermal activity, where hot, mineral-rich water facilitates the crystallization of minerals in rocks. This local geology allows for a diverse range of gemstones to be formed and is well-documented in gemological studies, making Ouro Preto a key locality for hydrothermal gems. This distinct characteristic of the region, combined with its historical importance in the gem trade, underscores its prominence as a source for such gemstones.