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Chert is a sedimentary rock made of silicon dioxide, SiO2. It has a microcrystaline structure, which means that it is composed of very tiny crystals. They form in chalky deposits of limestone.
This happens when water moves through the limestone carrying silica. Little by little the silica accumulates into nodules, concretions, and expansive layers.
Chert can be many colors, from black to white and nearly everything in between.
During the stone age chert was a favorite material for tool making. It is very hard, a 7 on the Mohs scale, and it can be broken to create sharp edges. It was used to make knives, scrapers, arrow and spear heads, axes, and many other tools, like the hand ax pictured above. It was so important that it may have been the first material mined.