eaglestone

from The Century Dictionary.

• noun A variety of argillaceous oxid of iron, found in masses varying from the size of a walnut to that of a man's head.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Min.) A concretionary nodule of clay ironstone, of the size of a walnut or larger, so called by the ancients, who believed that the eagle transported these stones to her nest to facilitate the laying of her eggs; aëtites.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun mineralogy A concretionary nodule of clay ironstone.