An Ioun stone is named after Ioun, a god of knowledge and prophecy revered on some worlds. Many types of Ioun stone exist, each type a distinct combination of shape and color.
When you use an action to toss one of these stones into the air, the stone orbits your head at a distance of 1d3 feet and confers a benefit to you. Thereafter, another creature must use an action to grasp or net 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.
Effect for this Ioun Stone: While this marbled lavender and green ellipsoid orbits your head, 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.