Chert is rock built from the glassy skeletons of microscopic sea life
Flint, the stone of prehistoric blades, is one form of chert, a hard rock made of quartz crystals too small to see. Most of it began as ooze on the deep ocean floor, piled up from the silica skeletons of diatoms and radiolarians. Some of the oldest chert even preserves fossil cyanobacteria.
Chemically, chert is almost pure silicon dioxide, usually with less than 5 percent other minerals, though impure nodules can drop below 65 percent silica. When it forms in chalk or marl, people call it flint. Its colours range from white to black, commonly grey, brown or green to rusty red; traces of iron make the reds in oxidised form and the greens in reduced form. Under the microscope it is mostly granular microquartz a few microns across, mixed with fibrous chalcedony and sometimes opal, though little opal survives beyond 60 million years.
The origin story explains its purity. Seawater typically holds only about one part per million of silica, far too little to crystallise out on its own. Living things do the work instead. Diatoms, radiolarians and glass sponges pull silica even from very dilute water, and together they are thought to produce around 12 cubic kilometres of opal a year. Diatoms can pluck silica from water containing as little as 0.1 parts per million and, in ideal conditions, double their numbers eight times a day. After death their skeletons normally dissolve, unless they settle and are buried as siliceous ooze, which later hardens and recrystallises into chert.
The best-known form is bedded or ribbon chert: layers from a few centimetres to a metre thick, separated by thin seams of shale, stacking up to hundreds of metres. The black shales often contain pyrite, a sign of oxygen-starved water, and the beds usually sit near deep-water limestones, undersea volcanic rock and the edges of colliding tectonic plates, places where little river silt reached the seabed.
Over time the style changed. Bedded chert dominates Precambrian rocks and becomes rare after the early Mesozoic, while nodular chert grew more common in younger rocks, even as the total amount of chert preserved in the rocks declined.
Source: Chert
