Age of oldest rocks off by millions of years - space - 29 March 2012. Agate Basics. This web page contains the following sections: Agates are semi-precious gemstones that are a variegated form of chalcedony (pronounced kal-sed'-nee), which is silicon dioxide in the form of microscopic fibrous quartz crystals.
Agates naturally develop when an empty pocket inside a host rock fills in molecule-by-molecule, layer-by-layer as these microcrystals self organize to form concentric bands or other patterns. The colors and arrangement of the microcrystals are influenced by changes in pressure, temperature, and mineral content that occur during the formation process. Unlike other gemstones, each agate is unique. Montmorillonite. Montmorillonite is a very soft phyllosilicate group of minerals that typically form in microscopic crystals, forming a clay.
It is named after Montmorillon in France. Montmorillonite, a member of the smectite family, is a 2:1 clay, meaning that it has 2 tetrahedral sheets sandwiching a central octahedral sheet.