Oco geodes are mined in northern Rio Grande do Sul province in Brazil. These geodes are quite popular given their smaller shape, thin skins, and wonderful agate in the shell with drusy quartz interiors. Many of these geodes also contain goethite crystals (or another related iron mineral). These geodes are approximately 138-127 million years old and are surface mined from outcrops in the Tres Pinheiros region of Rio Grande do Sul. Most Ocos are between 1-3 inches in diameter although some larger and smaller are also occasionally available. These geodes typically have a variable thickness agate shell with a fine drusy quartz lining of various colors from clear to light brown or smoky quartz most common. Some amethystine varieties are also encountered in approximately 5-10% of these geodes with the color varying from a light lavender to a dark smoky amethyst. All of our Oco geodes are imported directly from the mine owner in Brazil - the geodes were high-graded on site in terms of their hollowness and many of these geodes are considered "casa fina" (the most hollow geodes). All of our Oco geodes are guaranteed at least 90% hollow by volume, meaning at least 9 out of 10 will contain an open, crystal lined cavity. Please note on a few specimens, especially those where cracks were not detected, some dirt or rust may be present on the quartz lining. Such substances can be washed out of the geode once opened. Information source - "Geodes, Nature's Treasures" by Cross/Zeitner (2006) and Geode Gallery's supplier in Brazil.
Given the agate skins in these geodes, it is strongly recommended that these geodes be cut open versus cracked open especially if you want to polish the agate shell. They do take a very good polish when cut open and are among the most popular geode varieties to cut and polish given their banding and ability to take a good polish. The majority of these geodes are cut and polished on-site in Brazil - see our Opened Brazilian Oco Geodes page for already cut/polished Oco geode pairs.