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@New_DM_Tryingtobesneaky recently asked a question about damaging Ioun stones.
...and left this side note, which the answers don't address:
it's hard to believe that some rare/legendary items could be destroyed with a few save damage cantrips.
But rules-as-written, is it even possible to target Ioun stones this way?
No. Only creatures are subject to effects that require saving throws.
Spells that do damage based on a saving throw explicitly restrict their effects to creatures, including cantrips. Two examples of many:
Flame Strike:
A vertical column of divine fire roars down from the heavens in a location you specify. Each creature in a 10-foot-radius, 40-foot-high cylinder centered on a point within range must make a Dexterity saving throw. A creature takes 4d6 fire damage and 4d6 radiant damage on a failed save, or half as much damage on a successful one.
Burning Hands:
... Each creature in a 15-foot cone must make a Dexterity saving throw. A creature takes 3d6 fire damage on a failed save, or half as much damage on a successful one.
Ioun stones aren't creatures, so none of these spells (or any other spell that targets creatures) can affect them.
Some such spells also have specific effects against objects; two examples, again of many:
Fireball:
.. ignites flammable objects in the area that aren't being worn or carried.
Shatter:
A nonmagical object that isn’t being worn or carried also takes the damage if it’s in the spell's area.
Neither of these would affect an ioun stone; when in use, it's treated as being worn or carried, and when not in use, it's neither flammable nor nonmagical.
Ioun Stones specifically have an AC (24) so they can be targeted by attacks (spell or otherwise) that have an attack role. That's a special case.
For save vs DC attacks, there is no specific rule allowing them to be targeted with spells/abilities that typically can only target creatures. Ioun stones are considered worn equipment while circling your head, so the general rule applies, and Ioun stones cannot be damaged this way.
For area-of-effect spells, they follow the general rule that worn/carried items and are not damaged by area of effect spells, etc.
(Ref. Ioun Stone description - DMG p.177)
Certain spells, such as Chain Lightning, can specifically target objects.
