Portuguese for “hollow,” Oco Geodes are found in the steep hills of Tres Pinheiros in the state of Rio Grande do Sul, Brazil. Ocos can range in size from one inch up to five or six inches in diameter. Quartz crystals commonly line the inside of these spectacular geodes, and rarely the crystals are amethyst. Many of the Ocos contain black needle-like crystals (probably goethite) among the quartz. Ocos are easily distinguished by their “rope-like” surface on both the inside and outside of the geode. When cut and polished, the rims display prominent white wavy or flame-like patterns similar to their Trancas geode cousins.
An agate geode is a hollow agate lined with tiny quartz crystals. Agate geodes are formed in gas bubbles in lava. After the lava hardens around the bubble, water seeps through and deposits silicates and/or carbonates on the inside surface. Over time, this allows quartz crystals to form inside the hollow chamber. Bedrock containing geodes eventually weathers and decomposes, leaving geodes present at the surface. Agate geodes are found in Mexico, Brazil, Morocco and India.