cherts

Chert is a hard, compact sedimentary rock composed predominantly of microcrystalline quartz (silica, SiO2). Individual crystals are so small that the rock usually has a smooth, waxy or glassy luster and a conchoidal fracture. Chert occurs in a range of colors, including white, gray, yellow, brown, red, and sometimes green, and commonly forms nodules or beds within limestone and dolostone, or as layers within chalk deposits.

Chert forms by diagenetic processes in which silica is introduced to carbonate sediments, either by replacement of the original minerals or by precipitation of silica from silica-rich fluids. Much of the silica is derived from the remains of siliceous organisms such as diatoms and radiolarians, or from spicules of sponges, which accumulate and are altered to microcrystalline quartz. In chalk and limestone, nodules can become chert; the dark, opaque variety found in some chalk sequences is called flint.

Chert includes varieties such as flint (dark, opaque and commonly found in chalk), jasper (opaque and colored by iron oxides), and chalcedony varieties such as agate and other banded forms. Chert differs from other silica-rich rocks in its typical cryptocrystalline texture and its tendency to fracture conchoidally, producing sharp edges.

Historically, chert was widely used to make tools and weapons because its conchoidal fracture yields very sharp edges. In modern contexts, it is valued as a silica resource for glassmaking and as a decorative or aggregate rock. Chert also plays a role in fossil preservation, as silica replacement can preserve microfossils within carbonate beds.