A rusty, weathered, oxidized rock formation on the surface of mineral ore deposits that indicates the presence of ore below.
From Cornish gossan, from gos (blood) + an (stone), literally 'blood stone,' named for its reddish color from iron oxidation common in Cornish mining regions.
Gossan was the original treasure map—miners in Cornwall learned that whenever they saw this rust-colored rock outcropping, there was likely valuable metal ore beneath it, so the Cornish word for it literally means 'blood stone' because of its distinctive reddish hue.
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