Dodecahedron

The fifth regular solid — twelve pentagonal faces — is not assigned to an element. In the Timaeus the Demiurge uses it “for the whole,” embroidering it with the figures of the heavens: it is the shape of the cosmos itself, the solid closest to the sphere.

Its near-spherical roundness and later association with the twelve signs of the zodiac made the dodecahedron the emblem of the universe as a single living being — and, in time, a symbol of the quintessence or “fifth element.” See the Platonic solids.