 Depending on its origin, it can contain either microfossils, small macrofossils, or both.It varies greatly in color (from white to black), but most often manifests as gray, brown, grayish brown and light green to rusty red (occasionally dark green too); its color is an expression of trace elements present in the rock, and both red and green are most often related to traces of iron (in its oxidized and reduced forms respectively).Chert occurs in carbonate rocks as oval to irregular nodules in greensand, limestone, chalk, and dolomite formations as a replacement mineral, where it is formed as a result of some type of diagenesis.