The Necklace of explosive decapitation is the target of a lot of 'tricks'. Vast majority of them just don't work. Usually they are stopped by the fact the necklace only blows up when worn or carried.
This use at least is up to the DM.
Necklace of Fireballs
This device appears to be nothing but beads on a string, sometimes with the ends tied together to form a necklace. (It does not count as an item worn around the neck for the purpose of determining which of a character’s worn magic items is effective.) If a character holds it, however, all can see the strand as it really is—a golden chain from which hang a number of golden spheres. The spheres are detachable by the wearer (and only by the wearer), who can easily hurl one of them up to 70 feet. When a sphere arrives at the end of its trajectory, it detonates as a fireball spell (Reflex DC 14 half).
Spheres come in different strengths, ranging from those that deal 2d6 points of fire damage to those that deal 10d6. The market price of a sphere is 150 gp for each die of damage it deals.
Each necklace of fireballs contains a combination of spheres of various strengths. Some traditional combinations, designated types I through VII, are detailed below.
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If the necklace is being worn or carried by a character who fails her saving throw against a magical fire attack, the item must make a saving throw as well (with a save bonus of +7). If the necklace fails to save, all its remaining spheres detonate simultaneously, often with regrettable consequences for the wearer.
Moderate evocation; CL 10th; Craft Wondrous Item, fireball.
