By a closed diamond shape, we actually mean an octahedral rough diamond shape. Sometimes it is referred to as a diamond crystal with eight planes. Exactly how a crystal is formed depends on quite a few factors. There is the underground temperature for one and then there is the amount of pressure the crystal endures. A rough mineral could take the shape of a cube in one deposit and may take on an octahedral shape in another deposit. The internal structure however, always remains the same.