Six squares on twelve edges, three meeting at each vertex. Plato gives it to earth, because the cube is the stablest of the solids and the hardest to move, and earth is the element that holds. Of the five it is the one that sits flat, which is why the tradition uses it for foundation and for the built world rather than for anything that moves.
Sacred Ratio
1:square root of 3 (insphere radius to circumsphere radius)
Construction
Six equal squares folded so that three meet at every corner. Euclid constructs it inscribed in a sphere at Elements XIII.15, and the cube is the dual of the octahedron, so the centres of one give the vertices of the other.