Source: Dungeon Master's Guide

Armor (Any Light, Medium, or Heavy), Rare (Requires Attunement)

You have Resistance to one type of damage while you wear this armor. The DM chooses the type or determines it randomly by rolling on the following table.

1d10 Damage Type
1 Acid
2 Cold
3 Fire
4 Force
5 Lightning
6 Necrotic
7 Poison
8 Psychic
9 Radiant
10 Thunder