A Stone Grimace is an invincible stationary statue of a head, with a wide open mouth and hollow eyes.
They will sporadically spit a red bullet at Isaac's direction until the room is cleared from regular enemies, although they will sometimes attack 1 extra time after all the enemies are defeated.
Although indestructible, tear effects will still affect them, so it is possible to decrease their shot speed with Spider's Bite, or their range with items like Little Gish. It is also possible to petrify them briefly with Mom's Contacts.
They are considered regular monsters by Attack Flies or familiars like the Dead Bird, which might end up attacking one even though this will have no effect.
They cannot be passed over with Flight.
A green-tinted version of the Stone Grimace was added in the Wrath of the Lamb DLC. They can replace regular Stone Grimaces, and naturally appear in some rooms in Necropolis and Utero. Their behaviour is exactly the same as normal Stone Grimaces, but instead of firing a blood shot at Isaac, they fire an IPECAC shot instead. This shot's explosion can be used to kill other monsters in the room, break rocks, open Stone Chests, or open any Secret Room/Super Secret Room entrances.
When having cleared a room with Stone Grimace it is possible that one of them changes to a Vomit Grimace until leaving the room.
Gravity Grimaces were introduced in Rebirth and look similar to a normal Stone Grimace, but with a wider open mouth and circles radiating out from it. Instead of shooting a projectile at Isaac, Gravity Grimaces pull Isaac, Isaac's tears and enemy projectiles towards themselves. They can be found in the Caves and lower levels. Clearing the room does not deactivate Gravity Grimaces.
Also introduced in Rebirth, Cracked Grimaces will consistently spit bullets in a set direction as timed shots. Like Gravity Grimaces, clearing rooms will not deactivate them.
This type of Grimace was introduced in Rebirth as well. After short charging time, Devil Grimaces will shoot Brimstone-like beam in direction they are faced. Usually they can be found in Sheol. Same to another Rebirth Grimaces, they work even in cleared room.