Oco Geodes and Similar

They may be one of the most familiar of all the geodes, almost ubiquitous in any number of rock shops, trinket stalls, museum gift shops etc. - but that shouldn't blind us to what amazing things Oco geodes are. The delicate patterns in them and the complex crystal caves filled with everything from Amethyst to Goethite and beyond make them possibly one of the most remarkable rocks in the world when at their best - and I don't say that lightly.

I only recently became aware of the fact that there is possibly more than one Brazilian location involved. The classic Oco comes from Três Pinheiros, Fountoura Xavier, Rio Grande do Sul. These stones are dug out of the red-brown earth of the region and form a welcome extra 'crop' for the farmers. However, at least one of the following is probably something else, as you can see from the caption. And given the variety of these stones, I am left wondering whether there are any other locations mixed in here as well! For all their ubiquity, it is surprisingly hard to find information about these, maybe partly since nobody can agree on the exact name. I have seen Oco, Coco, Ocho, Ocos etc. However, Oco is the name given on the Mindat website, so I will go with that one for now.

They are a typically small agate, usually just a few centimeters long, but they can sometimes grow larger as you can see here. They have a tendency to form tear-drop shapes, presumably once gas bubbles. The agate layer is usually thin, even on the bigger stones.