Silver Has Mystic Powers

Silver was considered a mystical element in folklore, and thus its properties become reality in fiction. The metal can do many things, from channel magic, to stopping evil (including warding off or harming vampires and werewolves), making magic mirrors, to turning water into a Healing Potion.

The myth of silver's mystical properties goes deep into human history. As a noble metal akin to gold, this is often attributed to something along the lines of silver's Incorruptible Pure Pureness. In fact, it's because of something people noticed a very long time ago: if you put water in a silver pitcher, it takes a lot longer for it to get unhealthily scummy. Silver has antimicrobial properties that make it quite useful in medicine.

Due to the above, silver is almost always considered to be on the good end of magic (and items formed of it may also be Made of Good). Some works can make it evil though, or used for evil when Light Is Not Good. A Winter Royal Lady might prefer silver due to it fitting in with that trope's color scheme.

If Gold-Silver-Copper Standard is involved, expect Gold Makes Everything Shiny to be even more powerful, while silver still has its own powers.

A Super-Trope to Silver Bullet.

Compare Mithril (which is also called "silver"), Crystal Skull, Power Crystal.

Examples

In The Silver Chair, the eponymous item is used to brainwash a prince.
- And in The Magicians Nephew, the apples of immortality are made of silver.
The rat god, in Bride Of The Rat God by Barbara Hambly, can only be sealed away with pictures using silver in them. When the god uses his mind tricks to get a master painter sent away, the people trying to stop it remember that the silver in film could do the same thing (as this took place in Hollywood during The Roaring Twenties).
Dungeons and Dragons uses this a lot for so many items.
- It's possible to purchase weapons made out of silver, which do slightly less damage than normal (read: iron) weapons, but which bypass the damage resistance of Vampires, Were-animals, and some extraplanar creatures. Supplemental books provide an oil that can be used to coat a weapon and give it "silver" properties without making the weapon weaker, but it's hard to use while in combat (so you'll want to use it just before a fight starts, if you can).
  - Third edition calls it "alchemical silver", claiming that it's actually silver alloyed with steel so that it has only a -1 penalty to damage.
- Silver dust is a common material component of spells as well, ranging from "magic circle against..." to "bless water".
- The "Cloak of Lordliness" is a Pimped-Out Cape that grants powers to make crowds listen to you, and is made of ermine sewn together with silver thread.
- Forgotten Realms supplement Volo's Guide to All Things Magical. Silver is the metal most associated with and suitable for magic. Magic items that involve moon-related magics, electricity/lightning and energy discharges (e.g. Magic Missile) will automatically make all saving throws related to item creation magic if they're 60% (or more) silver by weight.
- Dragon magazine #32 article "Poisons from AA to XX".

Poison may be applied only to ordinary iron or steel non-magic weapons. Silver will not hold poison; the magic radiating from such weapons will burn it off.

From a Magical Metals list found on the Net

Silver: The metal of the moon, silver is well-known for being able to ignore lunar-based defensive powers such as those of lycanthropy. Unlike gold, silver will tarnish readily unless protected, but it needs no special preparation for most magical uses. Awakened, 3 ounces of silver (6 quantities for enchanting purposes) will grant moderate bonuses to divinations and magical wards it is used in. Unfortunately, silver is also strongly attuned to goodness and purity, it does not readily tolerate evil enchantments, having a marked tendency to decay into iron should the evil continue to be forced into it, though the presence of blue sapphire will aid the metal in resisting wickedness, and emeralds will weaken the silver if set in it. Like gold, but to a much weaker degree, silver can provide support for a spell, and also like its yellow cousin, it requires strengthening for physical reasons if used in weapons and armor. A wide selection of alloys can be made using silver, many exhibit varying degrees of tolerance for magical lightning, a quality the pure metal does not seem to have. Quite a few spells utilize the innate goodness of silver to strike out at evil, generally this does not harm the metal in the least.

In The Dresden Files, silver is pretty useful all around but ancestral silver is even better, i.e. it is the only thing that can kill the uber-werewolves loup garou. Harry has inherited a silver amulet from his late mom and it saves his butt at least once.
Conan the Barbarian stated that the only weapons effectives against demons are silver and fire. He later used a huge candelabra made of silver to smash a demon and make it fall in a fire.
In the LARP "The Realms," Silver is a weapon enhancement which is noted in the rulebook to have enhanced effects against certain types of beings.
In Moonlight it's mentioned that the reason vampires couldn't be photographed is because of the silver used in producing the negative, the silver reflecting the photographee's soul. (Which is also why they don't cast reflections in mirrors.) But modern digital photography and video don't use silver, making vamps easy to capture images of.
In a few of The Elder Scrolls games silver weapons (and daedric weapons, which were technically magical in nature) were the only way to harm ghosts without magic.
In Harry Potter, goblin-wrought silver is nigh-invulnerable, and can absorb the properties of what it pierces in order to make itself stronger. It's unclear how the goblins make it this way, or whether it's truly silver or simply called so because the same colour. There are many other objects in the series which are made of silver (the Pensieve) or have the appearance of silver (unicorn blood). Finally, several spells, most notably the Patronus, take a silver colouration.
Inverted in Mistborn, where silver is one of the few metals which doesn't have special powers.
Some of the levels in Super Mario Galaxy and Super Mario Galaxy 2 involved having the Power Star being broken up into five silver stars in which Mario must collect to complete the star.
In Rifts, Silver is one of the few things that can hurt nearly every supernatural creature in the game, up to and including many Gods. Consequently, nearly every PC in the game usually has at least a single silver-plated knife or cross. One nation at war with a kingdom of vampires equips Silver-plated swords on their Humongous Mecha.
The Manly Wade Wellman protagonist John the Wanderer carries a guitar with silver strings. This is because silver is the only substance in the world Satan fears, and as a consequence, it's proof against all beings of evil and darkness.
In The Witcher verse, witchers typically carry a silver sword to deal with monsters.