A fossilized shell of an ammonite (ancient spiral-shelled marine creature) that has been transformed into an iridescent gemstone, found primarily in Canada and prized for jewelry.
From 'ammonite' (the fossil) plus '-lite' (from Greek 'lithos,' meaning 'stone'). The term emerged in gemology in the late 20th century when these fossilized ammonites were recognized as precious opals.
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