The Bondage of Power
One of the most continually sought after applications of metallurgy, alchemy and magic in general was the negation of magic use in others. With spells not always requiring material components, a mage that was not bound and gagged could potentially be a threat. There were some powerful mages who could work their magic with thought alone. Ethics demanded a way to control such powers when the magi who possessed them broke the laws of man as well as the laws of nature.
A piece of magic-negating jewelry can either hinder a mages ability to work magic, hobbling him but not completely robbing him of his power, or it can be intended to completely cut him off from the ability to do magic. Most pieces that hinder magic power are typically voluntary in nature for magi who have more power than ability, while the completely negating pieces are more geared towards imprisonment and such.
Additional Ideas (2)
These silver and steel manacles were forged in recent years and serve a dual role, the physical restraint of the arms as well as a complete negation of a magic-user's ability to do magic. The metal of the manacles must be in direct contact with the skin to impede the working of magic, otherwise they are shiny and expensive bracelets. The manacles can be picked by mundane means, and they can be broken by force, though most mages tend to be on the scrawny side rather than the brawny. Magic users find the manacles cold to the touch and wearing them more than 24 hours causes a headache that gradually increases as long as the manacles are worn.
It comes to pass, from time to time, that there are magic users whose ability to channel magic far excedes their ability to control it. In more primitive says these poor souls would be made pariahs, cast out into the wilderness so that their raw power did little harm to their peers. In more enlightened times, the Amulet of Restraint exists not to cut off a mage's ability to channel power, but to limit it.
Most often a piece of jewelry, brightly colored, the Amulet of Restraint reduces a mage's ability to cast spells be a preset amount, with most Amulets reducing magic power by roughly 20% to 50% of it's maximum.