early 14c., from Old French diamant, from Medieval Latin diamantem (nominative diamas), from Vulgar Latin *adiamantem (altered by influence of the many Greek words in dia-), from Latin adamantem (nominative adamans) "the hardest metal," later, "diamond" (see adamant). Playing card suit is from 1590s; Sense in baseball is American English, 1875.
A form of pure carbon that occurs naturally as a clear, cubic crystal and is the hardest of all known minerals. It often occurs as octahedrons with rounded edges and curved surfaces. Diamond forms under conditions of extreme temperature and pressure and is most commonly found in volcanic breccias and in alluvial deposits. Poorly formed diamonds are used in abrasives and in industrial cutting tools.
Someone or something with potential or talent but lacking training or polish: “Her singing voice is beautiful, but she needs help with her gestures; she's a diamond in the rough.” This phrase refers to the fact that diamonds found in nature are rough and uneven. They must be cut and polished to bring out their true beauty.
A person of exceptional qualities or potential but lacking refinement or polish; a wonderful and surprising specimen: looked like a nerd, but rather was a diamond in the rough