This rare item is highly prized by those plagued by Psionic Monsters, as well as those fighting wizards who use Charms. They provide the effects of a Mind Blank spell, and also provide immunity to many other effects that would compel or control a person's mind. Further, they can be worn by any class, as their effects are automatic and uncontrollable by the bearer. They are fist-sized chunks of a greenish stone that glows when providing protection.
These items are quite powerful when up against these kinds of effects, but do have limits to the number of spell levels they can absorb. They recharge themselves, however, so are quite useful to provide some temporary protections from such effects. Mechanics aside, these items can be excellent options to give a player who is entering into a dungeon or area plagued with Psionic monsters. This would provide them with the chance to survive these encounters if they had no natural abilities in that area. My suggestion would be to have them either recharge more slowly than described in the entry (1 spell level per turn) or even not rechargable, thus to not make your party immune to most Psionic attacks. It would be easy to imagine a setting where they are actually common, and make them central to that setting. Protecting mining efforts by using the stones they are mining to fight off the creatures that would thrive in this kind of environment. In this kind of scenario, I would opt to have them be non-rechargeable unless some magical process is used to make them permanent. But leaving them in the environment will gradually recharge them over days or weeks. This makes them useful for these encounters in that specific setting, but almost useless in other situations. Making the setting difficult to reach would also help ensure these don't become a major force in your campaign.