3" Las Choyas "Coconut" Geode with Calcite & Agate - Mexico

Geodes are rounded, hollow voids in rocks filled with crystals and other minerals. They are typically formed when air bubbles inside of volcanic rock form hollow cavities. Over time, as mineral-rich water seeps into the rock it beings to deposit tiny crystals on the sides the hollow cavity. After millions of years, the flow of water gradually builds crystals inside the empty space.

Las Choyas geodes, often referred to as coconut geodes are mined from 100 to 200 feet below the surface near Chihuahua, Mexico. Shafts are drilled down to the geode bearing white clay, and then tunnels are dug horizontally to extract the geodes.

The geodes typically range from about 2-6 inches in diameter and can contain a variety of minerals and crystals. Most hollow geodes contain a variety of quartz ranging from clear quartz to smoky quartz to more rarely amethyst. Many secondary minerals such a goethite, hematite, mordenite, calcite and galena may also be present in some geodes.

Agate is a variety of microcrystalline quartz (chalcedony) that displays translucence and in some cases banding. Agate primarily forms when silica-rich fluids fill pockets within rock and/or fossils, resulting in deposition of the silica along the walls of the rock. This process can result in banding patterns as the composition and impurities of the fluids change over time. These banding patterns can either form as flat layers or rounded layers, depending on the surfaces available for deposition.

Silicon Dioxide, also known as SiO2 or Quartz, is the second most abundant mineral in the Earth's crust. Quartz crystals generally grow in silica-rich, hot watery solutions called hydrothermal environments, at temperatures between 100°C and 450°C, and usually under very high pressure. Quartz veins are formed when open fissures are filled with hot water during the closing stages of mountains forming, and can be hundreds of millions of years old.

Calcite, CaCO3, is a carbonate mineral and the most stable polymorph of calcium carbonate. The other polymorphs are the minerals aragonite and vaterite. Calcite crystals are trigonal-rhombohedral, though actual calcite rhombohedra are rare as natural crystals. However, they show a remarkable variety of habits including acute to obtuse rhombohedra, tabular forms, and prisms. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form.