Silver (metal)
For other uses of the word silver see here
To understand the significance of silver on the disc, it is best to begin with the fact the when Cheery Littlebottom arrived in Ankh-Morpork, among her possessions was a tunic of silver chainmail that had belonged to her granny and was something of a family heirloom. Why should a dwarf wear a silver mail tunic when this is a soft metal that would not easily withstand, for instance, being repeatedly hit with a battleaxe wielded by another Dwarf with malice aforethought?
The reason for this lies back home in Überwald. At the time of Cheery's leaving for the big city, an Agreement was in force between Dwarfs and werewolves which boiled down to You don't eat us, and we won't be forced to retaliate with the one substance that can seriously spoil a werewolf's day if used to plate spears, swords and axe blades.
Werewolves and silver do not get on. Witness Cheery's receiving the silver tunic, as while the Agreement was in force in Überwald, her family didn't need it, and nobody was sure that the werewolves in Ankh had heard of the Agreement nor would abide by it if they had.
Therefore Cheery wears it conscientiously every day (No thief in Ankh would ever countenance mugging an accredited axe-carrying Dwarf for her vest, however precious the metal it was made of) whilst confiding to Angua von Überwald that she still hadn't worked out who the werewolf was in the Watch.
In fact, the iron(y) law of narrative causality dictates that Angua, with the best of intentions (rescue) should, in her wolf form, have to be the one who is injured by biting the silver jacket... after rescuing Cheery from potentially becoming a very large ornamental candle, she slinks off with bad burns to her mouth and face, cursing all Dwarfs and their damn damn bloody silver underwear (Feet of Clay).
In another episode, Angua is trapped by 71-hour Ahmed, who recognizes her in wolf form and is quick enough to get a silver collar on her, so as to render her docile and dog-like. This has the effect, so near her brain, of fogging her thoughts and trapping her in her wolf-shape, as well as causing pain and discomfort (Jingo).
Until the events of The Fifth Elephant, the agreement between the dwarfs and the werewolves meant that no silver was allowed to be mined. However, by the end of the book, the Low King has re-opened the mines and presents Lady Sybil Ramkin with a ring made of the first ever silver mined in Überwald for decades.
Annotation
The obvious one: Dwarfs in ornate mailcoats fashioned from precious metal. This motif occurs in The Lord of the Rings, where Frodo inherits uncle Bilbo's dwarf-made mailcoat in mithril, an unidentified precious metal otherwise known as truesilver.