Marble is a decorative block commonly found in large underground deposits. Marble has a smoother texture than Marble Brick. Marble and Marble Bricks can be mined with a wooden pickaxe or higher, and mines noteably faster than Stone. The only current use for Marble is to turn it into Marble Bricks, and both are decorative blocks.
You can make a Marble Brick out of 4 Marble Blocks.