Cube (Hexahedron)

The cube — six square faces — is in the Timaeus the body of earth, chosen because it is the most stable and hardest to move of the solids, as earth is the most immovable element. Its squares are built from the isosceles right triangle — a different basic triangle from the one underlying fire, air, and water.

For that reason earth alone does not transform into the other three elements in Plato’s scheme: broken up, it can only be reassembled into earth. See the Platonic solids.