Flint

Flint is a sedimentary rock composed entirely of silica. It occurs as concretions, in band or nodule form, in limestones, especially chalk. It’s colour is usually black, brown or tan. It is marine in origin. Flint is a hard, tough stone which fractures conchoidally. 
Flakes of Flint were used by primitive people for making arrowheads, scrapers, and other tools.