Anti-magic iron

A special metal that messes with magic through the disruption of magic energy. To use magic, you effectively need magic energy that you shape through either purely intention or as an arcane caster intention + various tricks like hand signs, but this metal messes with the shape of spell energy causing the shaping to not turn out the way you want if you do the signs you normally do. As a result, it causes casting to be basically impossible, a skilled caster could practice to try and cast spells regardless of the anti-magic steel, but that is incredibly difficult to learn how to do those adjustments.

It's a rare material found in only a couple places on Gajas. An equivalent to anti-magic iron, called anti-magic steel can be made by mages working together with smiths (or doing the smithing) during the process of creating steel alloy. This however is a less common way to create the material because of its time and material cost being quite high.