Yes, the cube is earth for Plato. But for him, "aether" is a form of air (Timaeus 58d):Well, Timaios/Platon had cube (4) for earth (if I understood this correctly) and the Chinese heaven-metal is somehow opposite to the idea of "earth" (but also connected to 4 in China). And the dodecahedron with the pentagons (5) is associated to Aither (if I understood MikeH correctly) in Timaios/Platon and father/heaven/metal would fill the position perfectly, but is connected to 4 and instead mother/earth has the 5.
For Plato, and the dodecahedron is associated with the celestial realm of the zodiac, and not with any element. It is Aristotle who associates it with an element, which he apparently called the "first" element rather than "aether" (see https://en.wikipedia.org/wiki/Aether_%2 ... element%29). Aristotle is adding to Plato with this element, because of his need not to have a vacuum there. It was unlike the other elements, in not having any of the four qualities, in moving circularly rather than linearly like the other four, and was not subject to change. Wikipedia adds:In the next place we have to consider that there are divers kinds of fire. There are, for example, first, flame; and secondly, those emanations of flame which do not burn but only give light to the eyes ; thirdly, the remains of fire, which are seen in red-hot embers after the flame has been extinguished. There are similar differences in the air; of which the brightest part is called the aether, and the most turbid sort mist and darkness; and there are various other nameless kinds which arise from the inequality of the triangles. Water, again, admits in the first place of a division into two kinds; the one liquid and the other fusile...
But "aether", if referring to the substance of the "crystalline sphere"--a substance for which Plato apparently saw no need--is an Aristotelian contribution to tradition rather than a Platonic one.With this addition the system of elements was extended to five and later commentators started referring to the new first one as the fifth and also called it aether, a word that Aristotle had not used.
An example of this Aristotelian kind of thinking, four elements plus quintessence. applied to divination is found in Hugh of St.-Victor (1096-1141, a much-read author during the time of the early tarot (quoted without other particulars in Nigel Pennick, Secret Games of the Gods, 1989, reprinted 1997, p. 231):
Mantike, that is aeromancy, geomancy, hydromancy, necromancy, and pyromancy. The 'mancies' of the four elements, plus that of departed spirits, which might be associated with the Quintessence, and the alchemical arts.