diamond (n.)
early 14c., extremely hard and refractive precious stone made of pure carbon, 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 (n.)). Playing card suit is from 1590s; Sense in baseball is American English, 1875.