Say the party is able to attack a group of three creatures by surprise. The wizard decides to cast a fireball, which should kill a couple of the creatures outright. One of those creatures has an Ioun Stone of Protection (AC +1,) which is in his pocket. The creature uses the stone in combat, but keeps it in his pocket otherwise.
Will the fireball destroy the Ioun Stone (assuming the damage is 40 or more) while still in the creature's pocket?
A stone has AC 24, 10 hit points, and resistance to all damage types. (DMG p. 177).
The fireball thus needs to generate at least 40 points of damage to always destroy the stone.
[The fireball] ignites flammable objects in the area that aren't being worn or carried.
Apparently, a carrier confers a certain amount of protection to items he is carrying.