The stone is a dark blue sapphire, but its connection with the sapphire stone in Ezekiel, which played a great role in ecclesiastical allegory, was not known to the painter. The special virtue of the sapphire is that it endows its wearer with chastity, piety, and constancy. It was used as a medicament for 'comforting the heart.' The lapis was called the 'sapphirine flower.'
Jung identifies the sapphire hidden in the roots of the philosophical tree as an alchemical image of psychic wholeness, connecting it to Ezekielian allegory, traditional lapidary virtues, and the direct equation of lapis with the 'sapphirine flower.'
, Alchemical Studies, 1967thesis