Mom's Locket

Effects

Grants half a red heart each time a key is used to unlock a door, golden chest, or key block.
Turns half red heart pickups into full hearts. Half hearts already on the ground will also be upgraded upon re-entering the room.

Having a Golden Key or Paper Clip will render the trinket unable to grant half hearts from keys.
Having Mom's Locket while playing (Keeper) Keeper will restore a full health coin when a key is used.
Donating keys to a Key Master won't heal Isaac.
A very useful trinket for unlocking the (Added in Afterbirth †)(It's the Key) It's the Key achievement, in which Isaac cannot pick up coins, bombs, and most importantly hearts.
(Added in Repentance) Hearts dropped from (Tainted Magdalene) Tainted Magdalene's mechanic will be always full red hearts.
(Added in Repentance) With (Tainted Bethany) Tainted Bethany grants a blood charge upon using a key.

(Added in Afterbirth †) Equality!: As long as Equality is active, all red hearts will be double.
(Humbling Bundle) Humbling Bundle: Humbling Bundle cannot alter half red hearts, but Mom's Locket turns them into full red hearts, which it can double.