This shape allows the crystal to radiate energy in every direction at once. This is probably why a crystal ball is always been such a staple image when it comes to crystals. A crystal in the shape of a sphere has boundless potential and this is why they are so popular.
Crystal spheres or crystal balls are inherently connected to the earth, sun, moon, and stars. They are an excellent tool in helping to open the 3rd eye chakra which helps to increase intuition and clairvoyance.