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.
Awareness. You can't be surprised while this dark blue rhomboid orbits your head.
Notes: Detection, Warding
Why is this rare but a Weapon of Warning is uncommon, and the weapon of warning functions as a usable weapon, gives advantage on initiative, and has an aoe warning radius?
Yeah, not to mention you don't even need attunement to the weapon of warning—it just needs to be on your person. This ioun stone should probably be a straight up Common item in comparison.
Actually you do need to attune to the Weapon of Warning if you wish to use any of its effects. The fact that it just has to be on your person doesn't negate the attunement to the weapon