the name of a genus of fossils, distinguished from the flints and homochroa by their having a variety of colours. These are defined to be stones, composed of a crystalline matter, debased by earths of various kinds in the same species, and then subject to veins, clouds, and other variegations; usually formed by incrustations round a central nucleus, but sometimes the effect of a simple concretion, and veined like the agates, by the disposition the motion of the fluid they were formed in gave their differently coloured substances.