Estimated Value: 15,000 gp
The Mastery Ioun Stone is a pale green prism. You can use an action to toss this stone into the air, it orbits your head at a distance of 1d3 feet. While this stone orbits your head, your proficiency bonus increases by 1. Another creature can use an action to grasp or snare the stone to separate it from you, either by making a successful attack roll against AC 24 or a successful DC 24 Dexterity (Acrobatics) check. You can use an action to seize and stow the stone, ending its effect.
A stone has AC 24, 10 hit points, and resistance to all damage. It is considered to be an object that is being worn while it orbits your head.
Source: DMG p176