Silver's pairing with the Moon is, like gold's pairing with the Sun, one of the oldest and most consistently attested correspondences in the alchemical tradition, appearing across Hellenistic, Islamic and European sources with no significant historical dissent recorded by historians such as Lawrence Principe. Its alchemical symbol, a simple crescent, is identical to the Moon's own astrological glyph, again a direct visual expression of the correspondence rather than an arbitrary assignment.
Silver's slow tarnishing, in contrast to gold's near total resistance to it, gave alchemical writers a ready made explanation for why silver ranked second rather than first in the metallic hierarchy: it was closer to perfection than lead, tin, copper or iron, but not yet as stable, as fully matured, as gold itself, a graded scheme of relative perfection that modern chemistry does not recognize in those terms but that gave the alchemical tradition a coherent, internally consistent account of why the seven metals differed as they plainly did in their resistance to corrosion. Silver's continued role as a store of value alongside gold, in coinage and bullion alike, long after alchemical metallic hierarchies stopped guiding serious chemistry, shows how durable a practical, economic ranking of the metals proved even once its original theoretical justification fell away.