Ioun Stones fly 1-3 feet around the wearer's head, and are susceptible to attack (AC 24, HP 10, resistance to all damage), or being grabbed (DC 24 Acrobatics). It is considered to be an object that is being worn while it orbits your head. (Paraphrased, DMG p. 176, 177)
The HP & resistance information is listed after the AC initially, which leads me to think that they're not necessarily related, but it's hard to believe that some rare/legendary items could be destroyed with a few save damage cantrips.
The only save/damage spell that can target a nonmagical item is chain lightning (without looking through the Elemental Evil spells), so Ioun stones are significantly less vulnerable than I thought.