Carl Jung: The stone "is named the stone of invisibility"

The glass corresponds to the unum vas of alchemy (fig. 86) and its contents to the living, semi-organic mixture from which the body of the lapis, endowed with spirit and life, will emerge —or possibly that strange Faustian figure who bursts into flame three times: the Boy Charioteer, the Homunculus who is dashed against the throne of Galatea, and Euphorion (all symbolizing a dissolution of the "centre" into its unconscious elements).

We know that the lapis is not just a "stone" since it is expressly stated to be composed "de re animali, vegetabili et minerali," and to consist of body, soul, and spirit; moreover, it grows from flesh and blood.

For which reason the philosopher (Hermes in the ’Tabula smaragdina") says: ’The wind hath carried it in his belly" (fig. 210).

Therefore "wind is air, air is life, and life is soul."

The stone is that thing midway between perfect and imperfect bodies, and that which nature herself begins is brought to perfection through the art."

The stone "is named the stone of invisibility" (lapis invisibilitatis). Carl Jung, Psychology and Alchemy, Page 178.