first "Heh" of IHVH is associated both with Water and with the Briatic or Creative world....
Air is Vav of IHVH, and the Yetziratic or Formative world......
Earth is Heh final of IHVH, associated with the Assiatic world, the world of matter, where the creation which has developed through the other three elements achieves a relative stability. http://www.hermetic.com/browe/firetemp.html
Water, Air, Earth
Briatic, Yetziratic, Assiatic
There are 27 (cube of 3) pebbles altogether, of these 26 are visible while 1 in the center is invisible. 26 is the value of IHVH, Tetragrammaton, representing the Four Elements (Fire, Water, Air, and Earth) of creation. It is not unreasonable to proclaim the invisible center as corresponding to the Fifth Element, or Spirit.