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.
Reserve. This vibrant purple prism stores spells cast into it, holding them until you use them. The stone can store up to 3 levels worth of spells at a time. When found, it contains 1d4−1 levels of stored spells chosen by the GM.
Any creature can cast a spell of 1st through 3rd level into the stone by touching it as the spell is cast. The spell has no effect, other than to be stored in the stone. If the stone can't hold the spell, the spell is expended without effect. The level of the slot used to cast the spell determines how much space it uses.
While this stone orbits your head, you can cast any spell stored in it. The spell uses the slot level, spell save DC, spell attack bonus, and spellcasting ability of the original caster, but is otherwise treated as if you cast the spell. The spell cast from the stone is no longer stored in it, freeing up space.
Notes: Utility
Is there a way to mark what spells are stored in the stone?
When equipped and attuned should it display extra spell slots on the spells tab?
So this is the same rarity as a ring of spell storing, but holds less spells/slots, and has to be circling my head to use?
How does this work for ritual spells, and do the material components need to be expended twice?
Indeed.
Any idea why one would take this over the Ring of Spell Storing?
Druid Wildshape? Can't wear/use a ring in animal form, right? Depending on the DM, casting from an ioun stone floating around your head might be do-able.
I think animal form can still use rings as long as they've the ability to wear one, so, maybe not penguins, but wolves maybe?
You can attune to an Ioun Stone in a single action, rather than the typical 1hr short rest.
If there are some tactical considerations where you need to exchange spells between party-members quickly, the Ioun Stone of Reserve could do better.
For instance, let's say a 4 person party of 7th level needs to flee.
The Paladin already has used Find Steed on a previous day. They have 3 Level 2 Spell Slots, and want to use them to give the rest of their party steeds.
This requires either Ioun Stone of Reserve or Ring of Spell Storing.
With the Ioun Stone, it takes about 61 minutes total: 10 minutes to put Find Steed into the Stone, 10 minutes for a party member to cast the spell out of the stone, both repeated 3 times. Then a minute or so for spending a single action to attune and de-attune between the party.
With the Ring of Spell Storing, while it often is better, in this instance it is slower, because it would take several hours (at least 4, perhaps 7, depending on if you interpret "Any creature can cast a spell of 1st through 3rd level into the stone by touching it as the spell is cast." as meaning you can put spells into it without attunement, so the Paladin never has to attune to the ring).
You're probably better off with the Ring of Spell Storing if you have the option, but for some specific circumstances you might prefer the more agile nature of the Ioun Stone of Reserve.
"You can attune to an Ioun Stone in a single action, rather than the typical 1hr short rest."
You sure on that? Pretty sure it's "you have to use 1 hour to attune AND an action to toss the stone"
Because the DM would make it available earlier, or for cheaper.
This stone will be a very powerful addition to my players party, they don't even have access to level 3 spells, let alone the ability to save them.
Giving them the ring would be far too powerful for this group of level 3/4 adventurers. It also has hitpoints meaning it can take damage and eventually be destroyed, giving the DM a potential out.