"The vessel is always One, and it must be round
like the vault of heaven so that celestial influences
can contribute to the work. It is also often called a matrix or uterus,
in which the filius philosphorum is born,
and at the same time it is, in a mysterious way,
identical with its contents. For instance, it is simply the aqua permanens itself.
Mercurius is 'our true hidden vessel, and also the Philosophical Garden
in which our sun rises and ascends."
The Grail Legend ~ The Grail as Stone. p142. E. Jung & M-L Von Franz