From Ohio History Central
This sample of Brush Creek chert measures approximately 2.5" x 2.5" (6.35 x 6.35 cm).
Chert is an impure form of flint. Some people believe the opposite --- that flint is a variety of chert. In either case both minerals are varieties of quartz. The name chert is dialectal English; its origin is unknown. The mineral occurs in various colors including white, brown, yellow and gray. Chert is more brittle than flint and cannot be worked as well as flint.