Varies - see individual stones
Some say these enchanted gems take their name from a deity of knowledge, others that they are named for the sound they make as they gently orbit their bearer’s head. Whatever the case, there are many types of ioun stones, each imbued with powerful beneficial magic.
The specifics of these stones vary but they all work in essentially the same way: once you have attuned to it you can use an action to set a stone in a gentle orbit around your head, 1d4 feet away from you. Another creature may use an action to try and take an orbiting stone, but must make a melee attack against AC 24 or make a DC 24 Dexterity (Acrobatics) check. An ioun stone’s AC is 24, it has 10 hit points, and resistance to all damage. While orbiting your head, it is considered a worn item. You can use an action to seize and stow the stone, returning it to an inert and inactive state.
|Absorption||Very rare||10,000 gp||Refined negative energy||While this white marble sphere orbits your head, when a creature you can see casts a spell of 4th-level or lower targeting only you, you can use your reaction to have the stone harmlessly absorb the spell, suffering no damage or negative effects. The stone can absorb 20 spell levels. If a spell is higher level than the available spell levels left in the stone, the spell cannot be absorbed. After absorbing 20 spell levels the stone loses its magic and turns a faded gray.|
|Agility||Very rare||15,000 gp||A cat’s dream captured in the Astral Plane||While this ice-blue octahedron orbits your head your Dexterity score is increased by 2 (to a maximum of 20).|
|Awareness||Rare||600 gp||Phase spider’s eye||While this bright red cylinder orbits your head, it chimes to keep you from being.|
|Fortitude||Very rare||15,000 gp||Hoof or horn of a divine ox||While this bright pink dodecahedron orbits your head your Constitution score is increased by 2 (to a maximum of 20).|
|Greater Absorption||Legendary||50,000 gp||Refined negative energy, shards from a destroyed||This gold and onyx ellipsoid draws powerful magic into it. While this gem orbits your head, it functions as an ioun stone of absorption, but can absorb spells of 8th-level or lower, up to a maximum of 50 total spell levels.|
|Insight||Very rare||15,000 gp||Tail feathers from an awakened owl||While this clear octahedron orbits your head your Wisdom score is increased by 2 (to a maximum of 20).|
|Intellect||Very rare||15,000 gp||Canine teeth from an awakened fox||While this cobalt blue tetrahedron orbits your head your Intelligence score is increased by 2 (to a maximum of 20).|
|Leadership||Very rare||15,000 gp||Collar from a loyal dog that died of old age||While this brilliant yellow cube orbits your head your Charisma score is increased 2 (to a maximum of 20).|
|Mastery||Legendary||50,000 gp||The most treasured possession of a slain adventurer that redeemed themselves in death||While this jet icosahedron orbits your head, your proficiency bonus is increased by 1.|
|Protection||Rare||1,000 gp||Bone plate from a||While this dusty blue tetrahedron orbits your head, your AC is increased by +1.|
|Regeneration||Legendary||25,000 gp||gizzard||While this green and purple spindle orbits your head, you regain 15 hit points every hour, provided you have at least 1 hit point.|
|Reserve||Rare||600 gp||This radiant indigo prism encases a pearl and can store spells for later use. The stone can hold up to 3 spell levels and is typically found with 1d4 – 1 levels of stored spells.
To store a spell in the stone, a creature must cast a 1st-, 2nd-, or 3rd-level spell while holding it. The spell is drawn into the stone with no other effect. If there aren’t enough available levels to hold a spell, it cannot be stored.
While the stone is orbiting you, you can cast any spell stored in it using the appropriate action. The spell uses the spell level, spell save DC, attack bonus, and spellcasting ability of the creature that stored the spell, but you are considered to be the caster. Once a spell is cast, it is no longer stored.
|Strength||Very rare||15,000 gp||Canine teeth from an awakened bear||While this bright pink pentagonal trapezohedron orbits your head your Strength score is increased by 2 (to a maximum of 20).|
|Sustenance||Rare||525 gp||Honeycomb from a giant bee beehive||While this delicate amber spindle orbits your head, when taking a long rest you can roll a d20. On an 11 or higher, you do not require Supply during that.|