8.4" Colorful, Polished Agate Slab -"Big Dig" Site, New Mexico

Chalcedony is any microcrystalline variety of silica composed of very fine intergrowths of quartz and mogánite: microcrystalline minerals have microscopic crystals that cannot be observed by the naked eye. Both quartz and mogánite have the same chemical formula SiO2 (silicon dioxide), but different crystal structures. When free from impurities, chalcedony is colorless and transparent. Depending on impurities present during formation, chalcedony can form in a wide variety of colors including red, yellow, green, blue, purple, grey, white, and numerous hues in between. Chalcedony is quite hard at 7 on the Mohs Hardness Scale: this matches its main component quartz, which is the benchmark mineral for the scale at 7.