Ouro Prêto is a mining center, originaly for gold, later for gemstones, especially topaz. While the gold mines were located inside the borders of the city, the gemstone mines cover an area of about 20 square miles around the city. The city itself is today listed in the UNESCO Wolrd Heritage List for its wealth of historic buildings of the gold rush era. The whole city lives in its own history, a small collonial town, looking like a historic Portuguese town.
Ouro Prêto sits right on an outcrop of banded iron formation. This formation is a sedimentray rock with many layers of iron ore, often alterated to iron oxide especially ochre. There are also copper deposits, which sometimes are transformed to malachite. The dark rocks were the reason for its name Ouro Prêto which means black gold as a reference to the dark colour of the local gold, which contains a lot of iron oxides. However, the gold is mostly bound to gold bearing quartz veins, not to the iron.
The underground beneath the city is full of mine passage, there are kilometers of old passage, many are collapsed. Three parts of this enormous system are prepared for tourist, and allow a close insight into the mining history. Ouro Prêto has a mining school (Escola de Minas) with some tradition and a phantastic mining museum with an excellent mineralogic exhibition.
Today the area around Ouro Prêto is famous for its topaz. In fact, all pink topaz and most of the peach coloured so called imperial topaz mined in Brazil originates from an area less than 20 kilometers around Ouro Prêto. Hydrothermal lodes contain many mineral, among them quartz, hematite, magnetite, and accessory rutile, pyrite, and euclase. The enclosing rock was once schist or marble, but it eroded under tropic climate conditions to a lateritic clay. This chemical process does not harm the resistant gemstones despite its aggresivity, so they can be found in a good condition in the soft sediments.