Anyone else never see or use an ion stone at any point in their time playing this game?
I know they are a thing, and I include them in all judgments, but how many of us actually do have stones floating around us at all times?
Yep. There's a bunch of good ones just in core. They've long, long been good targets for a continual flame ioun stone (particularly burnt out gray ioun stones, which were priced at 50 gp in 3.0). In the core books, here's a few common goodies.
1. Dusty rose Prism (5,000 gp): +1 insight to AC. +5% evasion is good since it stacks with all your other shwag. Much cheaper to get one of these than to upgrade your +2-whatever to a +3.
2. Pearly white Spindle (20,000 gp): The HP regeneration from this sucks, however it otherwise works like a ring of regeneration which means it makes you immune to bleed effects and auto-regenerates any missing appendages, which may be desirable in some cases and is a cheaper way to get immunity to Bleed damage than a 90,000 gp ring.
3. Pale Green Prism (30,000 gp): +1 competence bonus on attacks, saves, skills, and ability checks. Might not sound like a great investment, but it's effectively +5% to everything, and honestly tacking on another +1 to hit and +1 to all saving throws in addition to your enhancement and resistances bonuses is definitely worth it.
4. Orange Prism (30,000 gp): +1 caster level. Yes.
Most are generally very expensive unless you craft them yourself, but some are cheap enough that you occasionally see NPCs with them as well. Especially dragons (though I'll usually describe them as being lodged in the dragon's scales with other gems and such, with no real mechanical changes).