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.
The colorless, transparent variety of quartz. Quartz is composed of silicon dioxide, the most common mineral in Earth’s crust after ice and feldspar. The name “rock crystal” emerged in the Middle Ages to differentiate it from colorless glass. The Greeks called water-color crystals krystalos, hence the name “crystal.” The name quartz comes from Old German, but its origins are unknown. Metaphysically, quartz is used for power, communication, and cleansing. Found worldwide, fine specimens of quartz can be found in Arkansas, Brazil, and Madagascar.