IAMOND HEX™ by Kadon

12 pieces made of 6 equilateral triangles joined ("hexiamonds") fill the circle and can make hundreds of other symmetrical shapes outside the tray, too. Toughest challenge: have all pieces of the same color connected, or separated. Proved by computer that each of these conditions has only one solution — out of the 55 possible ways to fill the 5½" tray.