As the better examples of Ioun Stones tend to be expensive, I was wondering about the ways you could prevent them from being stolen via the CM steal, or by attacks.
Ioun stones have AC 24, 10 hit points, and hardness 5. The powers of each stone vary depending on its color and shape.
There are wayfinders
All wayfinders include a small indentation designed to hold a single ioun stone. An ioun stone slotted in this manner grants you its normal benefits (as if it were orbiting your head), but frequently reveals entirely new powers due to the magic of the wayfinder itself.
The wayfinder suggests (but does not specifically state) that it protects the slotted ioun stone from the attack roles because it is no longer circling your head. Now it just becomes vulnerable to sunder attempts like other items you wear?
There is the possibility of having it implanted into a characters body which does prevent both steal and attacks made on the ioun stone. I find this an unattractive solution however.
I had a friend suggest having them made out of special materials like adamantine to increase its AC, hardness, or HP, but in my research I never came across anything like this. I do not know of a way to add enhancement values either which could also do similar things like that.
I considered having invisibility permanency on it, but only works when your opponent does not have see invisibility.
There is the amulet of grappling which helps prevents theft in adjacent squares. So would not help if a large creature was standing 2 squares away and tried to steal.
You could try to protect it by obscuring it by having lots of stones circling your head and a kind GM might give you a bonus for them to get the correct stone if they know about a particular effect or something.
There are the standard ways to increase your CMD as it applies to steal checks.
In the end I dont feel that most of the ways I have mentioned do a good job of protecting the stone. What other things could you do to keep a stone safe? Do not worry about the stone while you are sleeping.