Keystones are an environmental objects, found only in The Depths/Necropolis, The Womb/Utero, and the Cathedral. They block Isaac from reaching certain parts of the room, and cannot be destroyed. The only way to get rid of them is to walk over them, which will automatically consume a Key, if you have any. It is not possible to fly over Keystones.
- Opening a Keystone will sometimes drop a random Pick Up. Therefore, it can be worth opening them if you have an excess amount of Keys.
- In tight rooms, it is possible to unintentionally open Keystones, therefore, care should be taken around them if you do not want to spend any keys.
- The term 'keystone' is given to the top part of an arch, the part that completes it.
- It is not possible to use a key on a keystone when Isaac has the power of flight.