a mineral that is a complex borosilicate and hydroxide of aluminum containing iron and magnesium and calcium and lithium and sodium; it is usually black but occurs in transparent colored forms that are used as gemstones
"She carefully placed the rare, deep-green tourmaline into its velvet box to protect the precious stone before taking it to the jeweler."
A complex black or dark-coloured borosilicate mineral, compounded with various chemical elements and considered a semi-precious stone.
"The jeweler carefully sorted the rough tourmaline stones from the pile of other dark minerals before setting them into the ring."