Chert

Chert, an impure kind of flint, varying in colour, but generally black or bluish grey, occurring in nodules, as in the Portland beds, or in layers, as in the Greensand, and breaking with a flat, splintery fracture. Microscopic examination shows it to be often largely composed of much altered remains of Radiolaria (q.v.). In the Carboniferous Limestone are beds and nodules of a very flint-like black chert known as phthanite, which contains sponge-spicules and siliceous replacements of calcareous foraminifers and crinoids.