diamond (n.) Look up diamond at Dictionary.com
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.