Can you cancel a spell with Ioun Stone of Absorption if the spellcasting is imperceptible?

Question

Based on the wording of absorption (for the rod and ioun stones), I'm not sure if you can cancel spells that you can't perceive or not (spells requiring no components):

While this pale lavender ellipsoid orbits your head, you can use your reaction to cancel a spell of 4th level or lower cast by a creature you can see and targeting only you.

Counterspell requires that you be able to perceive a spell being cast, to react to it being cast, granted the wording of how counterspell works is slightly different:

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.

1. With counterspell, you interrupt a spell during casting, but with absorption, you react to a spell once it has been cast, if it targets only you; at least that is my interpretation of the wording.
2. The wording of absorption does require that you see the caster, which implies that you can't simply react to the spell targeting you alone without some kind of prompt (if you couldn't see the caster).

I'm completely split on whether you should be able to use the ioun stone of absorption to cancel spells that don't use components, but I can't find any rulings on it.

Answer 1

The stone doesn't have the same limitations as Counterspell

Whenever a spell has a casting time of 1 Reaction, it always has an asterisk describing when you can take that reaction. The reaction timing for Counterspell is:

when you see a creature within 60 feet of you casting a spell

This is the part of the spell's text that requires the target's spellcasting to be perceptible in order to counter it. As you've noted, there is no corresponding limitation in the reaction timing described for the Ioun Stone of Absorption. As long as you can see the caster and the spell is targeting only you, it satisfies the requirements for the reaction.

As for the fact that the stone can only be used if you can see the caster, the logic for this is not clear to me. It could just be an arbitrary limitation for balance reasons (e.g. preventing you from absorbing a Scrying spell cast from hundreds of miles away). In any case, the most straightforward reading of the item description only requires the user to see the spell's caster, not its casting.

Incidentally, there's no reason a priori to expect the two to have the same reaction timing or requirements. Counterspell interrupts a spell during the casting, while the Ioun Stone of Absorption absorbs the spell's energy after the casting is complete and that energy is released.

As an additional note, absorbing a spell in this way also avoids other limitations of Counterspell, such as range and line of effect. Last session, I was battling an enemy mage who had, unknown to me, set up a Globe of Invulnerability and then targeted me with a spell while inside it. If I had tried to counter the spell, it would have failed automatically since the globe would have blocked my line of effect to the caster. Fortunately, I had a Rod of Absorption and chose to use it instead. Since there is no line of effect requirement, it worked, and the spell's effect was negated. Of course, the next round he cast a Cone of Cold, and instead of countering it I tried and failed to absorb it, because it's not a single-target spell. So the point is, spell-absorbing items have different limitations from Counterspell.