Stone of Concentration (3.5e Equipment)
From D&D Wiki
This item resembles a finely polished stone, with a single hole graven into it. When focusing on it for a mere 1d4 minutes at the start of the day, it bestows upon the user a competence bonus to Concentration checks equal to the stone's enhancement bonus for the next 24 hours. To continue benefiting from the bonus for the day, the stone must be somewhere on the owner's person.
Prereqs: Craft Wondrous Item, bear's endurance. Caster level must be at least equal to the stone's enhancement bonus.
Cost to Create: 800 gp (+2), 3,200 gp (+4), 7,200 gp (+6), 12,800 gp (+8), 20,000 gp (+10).