Eagle-stones. (Greek, aetos, an eagle.) Hollow stones composed of several crusts, one within another. Supposed at one time to form part of an eagle's nest. Pliny mentions them. Kirwan applies the name to clay ironstones having a globular crust of oxide investing an ochreous kernel. Mythically, they are supposed to have the property of detecting theft.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894