In natural history, a sub-species of quartz, of a yellowish or bluish gray, or grayish black color. It is amorphous, interspersed in other stones, or in nodules or rounded lumps. Its surface is generally uneven, and covered with a rind or crust, either calcarious or argillaceous. It is very hard, strikes fire with steel, and is an ingredient in glass.

A piece of the above described stone used in firearms to strike fire.

Any thing proverbially hard; as a heart of flint.