What is the secret of the black cube and Saturn according to the original Freemasons? Without diplomacy

The cubical stone has occasionally been adopted as a symbol of a Masonic lodge, reflecting older symbolic traditions in which cubical geometry was associated with deity and truth 1. In ancient accounts recorded by Masonic writers, Mercury was represented by a cube to signify truth, while an altar of cubical form dedicated to Apollo gave rise to the classical legend of doubling the cube 1. Additionally, traditions in Arabia attributed occult virtues to a black stone shaped as a double cube 1.

The library sources contain no references connecting Saturn to a black cube, nor do they record any secret doctrine regarding Saturn among early Freemasons.