Sedimentary rock

Radiolarite

A hard, fine-grained, chert-like siliceous rock made mostly of radiolarian skeletons (opal-A). Most are deep-water pelagic deposits; those without carbonate likely formed below the carbonate compensation depth. Colors range from white to black via red, green and brown. Known from the Upper Cambrian onward.

In thin section

Radiolarian tests: silica spheres with low birefringence, like chert.