Oco Geode Flats Uniqueness Guaranteed: Every item is one-of-a-kind, making them ideal for all projects or designs. Listing is for (1 Flat) Avg Size Range: (1 to 2 ) Sizes Vary Oco geodes form when groundwater deposits sediment and other minerals like quartz inside basalt pockets, which are then sealed over. These minerals harden in a similar process to agate, and the basalt eventually wears away to leave the geode behind. When cut open and polished, these geodes display colorful wavy patterns along the rim. Custom Requests: Have specific preferences? Leave a message with your order, and we'll do our best to accommodate your requests if possible. Handmade items vary in size, shape, and color when selected randomly (the highest quality items are chosen first). They'll be as beautiful as those in the photos.