Silver is the metal of the Moon in the classical planet-metal scheme, the counterpart to solar gold, and the pairing is nearly as intuitive: silver is white and reflective like moonlight, cool where gold is warm, and second in value where gold is first. The alchemists gave silver and the Moon the same symbol, a crescent, and called the metal Luna.
In the great opposition that structures alchemical thought, Sol and Luna, gold and silver, Sun and Moon, king and queen, the fixed and the volatile, the masculine and the feminine, must be joined for the work to succeed, so silver is not merely inferior gold but its necessary complement. Silver's reflectivity gives it a symbolism of mirrors, of reflection and intuition, and its association with the Moon links it to the tides, to cycles and to the feminine principle. Ritually and in folklore silver was protective, held to repel evil and later to be the metal fatal to werewolves; it also has genuine antimicrobial properties known long before their cause was understood.