Chert is a fine-grained siliceous rock, commonly found as fist-sized nodules embedded in a limestone matrix. When you break it, it fractures smoothly in a conchoidal pattern -- in rounded flakes. It was used by people all over the world to make stone tools.

Flint, jasper, and agate are all varieties of chert. Plain brownish chert is simply called chert. Flint is dark grey or black. Jasper and agate are used as semiprecious stones for their banded or flowery crystalline patterns.