The Probert Encyclopaedia of Rocks & Minerals

CHERT

Chert (hornstone) is a mineral very similar to flint, but coarser and less uniform in colour. It is found principally in association with limestones, especially in the carboniferous limestone of Ireland where beds of it are found several hundred feet thick. It appears to have resulted from the solution and redeposition of the silica of certain kinds of fossils, particularly of sponges, with the pointed spicules of which it is often filled. Radiolarian chert is a streaky, dark-grey, brown or reddish rock which under the microscope is seen to consist of innumerable shells of Radiolaria firmly united together by a siliceous cementing material.
Research Chert