Removes all current Red Heart Containers. Does not remove existing Bone Hearts, but does empty them.
Upon losing a Bone Heart, including ones acquired from other sources, Isaac fires a ring of 8 bones that split into more bones.
Each Bone Heart lost adds a permanent +0.5 tears up.
Each Bone Heart lost adds a permanent +0.4 fire rate, which can break the tear fire rate cap.
Each bone does twice Isaac's damage plus a flat 22 damage; split bones do half this amount.
Notes
This item has a 1% chance to drop when defeating The Frail.
If Brittle Bones is removed from Isaac's inventory, its tear upgrades will not be removed but losing further Bone Hearts will no longer have an effect.
Picking this item up as a character unable to gain Bone Hearts will grant the stats equivalent to one Bone Heart breaking.
The tears up bonuses gained through this item will persist in the escape sequence of Mines II.
Losing Bone Hearts not from taking damage (e. g. through a Devil Deal or if a Bone Heart is replaced with a Broken Heart) will not give the tears up effect.
Synergies
Empty Vessel: The empty Bone Hearts will trigger Empty Vessel's effects.
Found Soul: The soul does not benefit from the tears up granted to Isaac. However, it will receive a fire rate increase if it somehow receives Bone Hearts and breaks them.
Bone Hearts.
Red Heart Containers. Does not remove existing Bone Hearts, but does empty them.
Each Bone Heart lost adds a permanent +0.5 
This item has a 1% chance to drop when defeating 
The Frail.
Keeper 
(and 
Tainted Keeper) will also gain 24 Blue Flies.
Mines II.
Empty Vessel: The empty Bone Hearts will trigger Empty Vessel's effects.
XIX - The Sun?: Using the card before picking up Brittle Bones allows Isaac to keep the heart containers since they will be temporarily converted to 
Genesis: Removes the tear upgrades upon use.
Found Soul: The soul does not benefit from the tears up granted to Isaac. However, it will receive a fire rate increase if it somehow receives Bone Hearts and breaks them.
Concept art for Brittle Bones on the left side, along with other Booster Pack items and trinkets, by Edmund McMillen.
Bones 
