Estimated Value: 31,000 gp
The Insight Ioun Stone is a marbled lavender and green ellipsoid. 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, you can use your reaction to cancel a spell of 8th level or lower cast by a creature you can see and targeting only you. Once the stone has canceled 50 levels of spells, it burns out and turns dull gray, losing its magic. If you are targeted by a spell whose level is higher than the number of spell levels the stone has left, the stone can't cancel it.
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