Dimeritium

Journal Entry
"Dimeritium is a rare and precious metal with an interesting feature - it represses the transfer of magical energy. A bluish alloy of iron and dimeritium is used to produce handcuffs and necklaces. Those who wear them cannot cast spells or use magic in any way. There are known cases, however, of extraordinarily powerful sorcerers managing to overcome the shield generated by dimeritium."

Source

 * Tower Mage's Book

Trivia

 * Triss Merigold gave Alvin a dimeritium amulet to help the boy.
 * Jacques de Aldersberg wears a dimeritium amulet under his armour and clothes.