A painful skin infection with multiple pustules that is more severe than a boil, or in gemstones, a deep red garnet or ruby.
From Latin 'carbunculus' meaning 'little coal,' from 'carbo' (coal), because the infection looks like a glowing coal. Used for both medical and gem contexts since medieval times.
Medieval doctors thought carbuncles were caused by burning humors in the body, so they'd actually try to 'cool' them—the name comes from the coal-like appearance that made people think they were literally burning from the inside.
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