Silver is the Moon in the alchemical scheme, and its symbol is a crescent. In Latin texts the metal is often simply called Luna, and the association is one of the most stable in the whole tradition, attested from late antiquity and unchanged through the medieval and early modern corpus.
The grounds for it are more obvious than for most of the seven. Silver is white and reflective; the Moon is white and reflective. Freshly polished silver is the brightest of all metals, reflecting more visible light than any other surface, and the mirrors of the ancient and medieval world, where they were not polished bronze, were silvered. The Moon is second to the Sun in the sky, and silver is second to gold in value. The correspondence is a straightforward reading of appearance and of rank, and it requires no hidden doctrine.
The vocabulary of the laboratory kept it. Lunar caustic is silver nitrate, so named because it is made from silver and because it burns tissue; it was a standard cautery and antiseptic into the twentieth century, and the name is still occasionally heard. Horn silver is silver chloride. The tincture of the Moon in alchemical medicine is a silver preparation.
Silver also has a real association with purity that is not merely symbolic. It is antimicrobial: silver ions disrupt bacterial cell function at very low concentrations. Water and wine kept in silver vessels genuinely spoil more slowly, and the practice of using silver for storage and for eating utensils, common in wealthy households across many cultures, had an effect its users could observe without being able to explain. Silver dressings and silver-impregnated wound coverings remain in clinical use. This is one of the few places where a traditional attribution of a property to a metal turns out to be literally correct.