There are hundreds of areas around the world where geodes of various types can be found in abundance. Most of these deposits are small and support the collecting activities of a few rockhounds. However, other deposits are extensive, with enough geodes to support a commercial collection and manufacturing industry.

Oco (Ocho) Geodes

Oco or Ocho geodes are small agate geodes with a drusy quartz lining that are found in the Tres Pinheiros region of Brazil. They range in size between about 1/2 and 3 inches in diameter and formed in the vesicles of basalt flows that underlie parts of this region. Most Oco geodes have a thin agate rind, an open interior, and an interior druse of tiny sharp quartz points about 1/8 inch long. Upon weathering, the basalt flow forms a reddish-brown soil and the geodes, being much more resistant to weathering than the basalt, accumulate in the soil.

When the deposits were first exploited, the geodes were easy to find and became a local business for people who collected and sold them. Many Oco geodes are sawn in half and polished, or are cut into slices and polished. These are sold in rock shops and novelty stores to people who enjoy interesting rocks and crystals. Because many Ocos have a very thin rind, they are often packaged and sold as “break a geode” kits. These are a popular activity for elementary students who are learning about minerals and crystals in a science class.