There is one more group of shapes, which while they are man-made, the shape is determined by the minerals. If you break a stone, different things happen depending on the stone. It may break very easily along a particular plane. This is called cleavage. Slate can be split very easily into thin slabs, which is why it is used for roofing.
Calcite has two cleavage planes.
If you break a piece, you tend to get smooth surfaces parallel to each other without being a cube. You can see this in several of the specimens on the right. These are not crystals, it is just the way that the stone splits. Other minerals don't split so easily. The surface caused by the break is not smooth and flat.