From Dwarf Fortress Wiki
Chert is a brown stone found in sedimentary layers. It is magma safe, and otherwise unremarkable. In the real world, flint is high-quality chert, and it has been used for stone tools as it chips with sharp edges.
[INORGANIC:CHERT] [USE_MATERIAL_TEMPLATE:STONE_TEMPLATE] [STATE_NAME_ADJ:ALL_SOLID:chert][DISPLAY_COLOR:6:7:0][TILE:'='] [SEDIMENTARY] [IS_STONE] [MELTING_POINT:13101]