clay ironstone

Ashfield Shale comprises black mudstones and grey silty shales with frequent sideritic clay ironstone bands.

The American Geological Institute defines the eaglestone as "a concretionary nodule of clay ironstone about the size of a walnut that the ancients believed an eagle takes to her nest to facilitate egg-laying."

For example, the OED records its use in 19th century poetic diction to describe flowers, "a variety of clay ironstone of the coal measures", "the colour of raw silk", and a breed of ray.