There are infinitely many possible answers that depend on the relative shapes and sizes of the diamonds.
a cube
Sorry I don't think it makes one
yes ithink
no
Diamonds can be cut into almost any shape.
Do you mean diamonds as in diamond rings, or as in a pack of cards? If the crystal version, then you can't do anything with them other than by grinding bits off to change the shape.
Octahedron
no
As found in their natural state, any
Three rhombuses can be arranged to form a hexagon. A rhombus is a quadrilateral with all sides of equal length, so when three rhombuses are placed together, their sides can form a closed shape with six sides. The angles between the rhombuses will be 120 degrees, creating a regular hexagon.
Some raw diamonds appear to be pyramids, or a pair of pyramids joined at the base. The edges are not sharp and these descriptors only approximate the shape of the natural stone. You can follow the link, below, to learn more about raw diamonds.
All diamonds are formed from carbon.