1 : a very hard usually colorless stone that is a form of carbon and is used especially in jewelry [count] ▪ a necklace studded with diamonds and rubies ▪ Her husband gave her a diamond [=a diamond ring] for her birthday. [noncount] ▪ The ruby was surrounded by a ring of diamond. — often used before another noun ▪ a diamond mine/ring — see color picture 2 [count] : a shape that is formed by four equal straight lines and that has two opposite angles that are smaller than a right angle and two opposite angles that are larger than a right angle
▪ The children cut the fabric into diamonds. — compare square 3 a [count] : a playing card that is marked with a red diamond shape
▪ one heart, two diamonds, and two clubs — see picture at playing card
b diamonds [plural] : the suit in a deck of playing cards that is marked by red diamond shapes
▪ He played the queen of diamonds. — compare club, heart, spade 4 [count] a : the part of a baseball field that includes the area within and around the three bases and home plate : infield
▪ The infielders warmed up by throwing the ball around the diamond.
b : the entire playing field in baseball
▪ It's sad to see the town's old baseball diamond deserted.