A type of nodular stone or mineral concretion, especially one found inside another rock and making a rattling sound when shaken; also called eagle stone.
From Greek aetos (ἀετός) meaning 'eagle,' as these stones were believed to contain smaller stones that made noise, like eggs in an eagle's nest. The name reflects ancient beliefs about their origin.
Medieval people found these hollow stones that rattled when shaken and were *convinced* they were magical eagle eggs—they wore them as charms for fertility and safe childbirth, not realizing they were just geological accidents where minerals crystallized around an empty space.
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