From Witcher Wiki
Dimeritium is a special sort of metal which apparently blocks magical power. Vilgefortz of Roggeveen, together with other Chapter members, was shackled with dimeritium after he was found guilty of treason. It was stated that being shackled causes unpleasant side-effects for the mages.
| "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."
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