When dropped, creates an expanding red sphere that will explode with the message "KABOOM!" when entered by a creature or character, causing heavy damage (40+). The explosion can pass through doors and walls. A firestone can be disarmed by picking it up before it detonates. It is unaffected by wands of cancellation once dropped.
Sometimes dropped by Acid Blobs.
Caution! If the character is more than a few steps away from a firestone when it is dropped it will likely detonate before it can be picked up.
If a character finds themselves trapped between a dead end and a firestone that cannot be safely recovered, it is best to rest until a monster appears and detonates the firestone to remove the obstacle.
A pet can be sent to fetch the firestone. It will detonate and probably kill the pet, but it will clear the way for you to proceed.