Crystal Armor

You create an armor of glistening crystals on your body.  This armor increases your AC by 1 for the duration.  If a creature hits you with a melee attack while you have this armor, the creature takes 1d8 piercing damage.  A creature can take this damage a maximum of once per turn.

At Higher Levels.  When you cast this spell using a 2nd level spell slot, the piercing damage increases by 1d8 for each slot.