Chert

This is a short overview. For the background — history, value over time and notable mines — see Chert →

A hard, dense, cryptocrystalline form of silica occurring as beds and nodules in limestone. Too brittle for building and too hard to shape easily, its industrial value lay in that hardness: blocks of chert were used as the grinding medium in pottery mills to reduce flint to a fine white powder without contaminating it, and the Pennine chert quarries existed almost entirely to supply the Staffordshire potteries.