carbuncle
Carbuncle (noun) means a dark red gemstone, such as a garnet, especially one polished into a smooth rounded shape rather than cut with facets.
Etymology
From Middle English carbuncle, charbocle, from Old French carbuncle, charbuncle, from Latin carbunculus ("a small coal; a reddish kind of precious stone; a kind of tumour"), a diminutive of carbō ("a coal, charcoal"). It is a doublet of Garfinkel and Garfunkel.