Chert is a brown stone found in sedimentary layers. It is magma safe, but otherwise unremarkable.
 In real life
Chert occurs as oval to irregular nodules in limestone, chalk, and dolomite formations. A specific, high-quality type of chert known as flint is used for stone tools as it chips with sharp edges. The art of sharpening flint to create weapons such as arrows is known as knapping.