Your friend has a cube of gold. You have a cube of gold that weighs twice as much. Your cube is as long as your friends, and it has times the surface area. You can imagine how this would work with a spherical diamond, so we won't repeat that part here. Here is another question: The ratio of the weight of your cube and the surface area of your cube is times as large as the corresponding ratio of your friend's cube.
Hint: Understand scaling.