Smuggler's Locket (3.5e Equipment)
From D&D Wiki
A Smuggler's Locket is a pendent with a large, hemispherical precious stone attached to a gold background. The necklace has a secret clasp (Search DC 22 to discover) which opens the locket. The inside of the stone is actually hollow, which allows for a small compartment. The compartment can hold a very small item (or several such) in its approximately 1 cubic centimeter volume. The locket is often used by all manners of brigands in disguises, from smugglers who want to secretly transport jewels to assassins who carry powdered poison in it when they attend social functions. Because the large stone is actually just a thin shell (and is hollow), the pendent is not worth nearly as much as it seems. The stone's value can vary (anything from emeralds to topaz have been used), so the price below is only an estimate.
Cost: 25 gp