The obsidian ioun stone increases your constitution by +1. This will increase your health in the logical way; you retain the same percentage of your max health. However, unequipping the stone does not reduce your health in any way whatsoever. Practical use? Infinite free healing through equipping/unequipping over and over. Bioware should really not have let this one slip through.
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because you're worth it
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Quite a few items in the game are like that. Sensate amulet and spear of Khuldahar comes to mind, also Barbarian rage. I agree that they should have tried to fix that for ToB.