The volume of the granite piece is 2.2 (2.2 = 17.8-15.6) cubic centimeters. A graduate cylinder is a tool for measuring a volume of a liquid object. The granite piece's volume could be obtained by measuring the difference between the beginning of water volume and the latter water volume assuming there is no reaction between the water and the granite piece. The cubic centimeter unit is equal the milliliter unit, so there is no conversion needed.