Mirror Scale (5e Equipment)
From D&D Wiki
Armor (scale mail), uncommon (major tier)
Durable armor, made from the glass-esc scales of the mirrorwyrm. While not incredibly effective armor, it does carry a degree of the magic resistance the wyrm itself has to lower end magic. Rather popular with Aldthar, as the potential to better repel wizards is very enticing.
This material as armor is only really popular with outsiders, as the natives hardly had any contact with magic, however it has seen usage in decoration.
Magical Resistance. When you take damage from a spell cast at 3rd level or lower, you reduce the damage you take by 7. This is reduced before resistance and vulnerability.