Agate - LSA7.92
Agate, Lake Superior
Agates are a microcrystalline variety of quartz known as chalcedony. They are distinguished from other varieties of quartz by color bands that are often concentric. Lake Superior agates formed about one billion years ago when silica-rich fluids filled in hollow cavities (vesicles) of molten rock (lava) created during volcanic activity. The concentration of iron and the amount of oxidation determine the color within or between an agate's bands.