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.
Greater Absorption. 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.
Notes: Warding, Consumable
So, why is this one consumable and the Rod of Absorption (which can absorb 9th level spell) isn't consumable and a rarity lower?
Okay, so I know it's like, a couple month's late, but here's the reasons:
1.The Rod of Absorption is consumable. This is important, and easy to miss when you first read the stat block. Note the exact language:
The ability to use the Energy in place of spells does not lower the number of spell levels absorbed over the course of a lifetime. You get 50 levels worth of uses, that's it. So it actually is consumable.
2. The rod requires you be holding it. RAW, This is kind of a bigger deal than you might expect. This means that weapon users have to give up their shield or two handed weapon in order to use it. This, I expect, is the main reason for the rarity difference. The Ioun stone allows a Martial character, such as a Greataxe Barbarian or Sword and Board Fighter to cancel spells without any loss in Defense or Offense.
Additionally, it is not explicitly a general spellcasting focus, which means that Clerics, Paladins, Druids, Rangers, and Bards cannot use it as a spellcasting focus, and thus must have their other hand occupied with a spellcasting focus or nothing at all. If they hold a spellcasting focus, they can't cast spells that don't have a material component (such as Shield and Counterspell). If they don't hold a casting focus, then they lose out on any bonuses they might get from their spell casting focus (which you would expect is probably a magic item of it's own by the time you could get a Very Rare / Legendary item).
4. The Rod of Absorption also just automatically starts with less levels of absorption (50 - 1d10 left) than the Iuon Stone, which is assumed to have a perfect 50.
Does all this justify it being a legendary item? Honestly, no, but all the Iuon stones are overrated in the terms of rarity. I suspect that the designers expected that having free hands would matter more in terms of Magic Item economy than it ended up being, since Attunement is such a chokehold anyway.
No you no
Why does it say 1d3 instead of 1d4?