Ioun stones are an enchanted stone common in fantasy fiction (they actually originated in the Dying Earth series by Jack Vance), and in role-playing games.
They can appear in a variety of shapes and colors, but spheres and prisms are most common. They are generally about the size of a marble, and may be shiny, dull or even transparent. These stones act as little satellites around the owner's body, continually orbiting semi-randomly within a few feet of the wearer's head.
Now Ioun stones are not just good for their fashionable orbit patterns. They impart magical powers upon the wearer. Different stones have different effects, but they can increase the wearer's natural physical and mental abilities and even their learned skills. Some stones can sustain the wearer without food, water or even air. While others can help the wearer to heal his wounds at an unearthly rate. A final class of these stones (and the most common originally made), serve to absorb or store magical spells and abilities. This last type of stone will eventually burn out and turn grey, but it will continue to keep its orbiting ability, even if it has no effect.
To use a stone's power it must be orbiting your body. They can simply be released from your hand, and they will take up orbit around the nearest body (the owner's, unless she meaningly sends them into orbit around someone else).
It is possible to grab someone's Ioun stone and take it away, but it is difficult because they do not move in any sort of predictable pattern. So you have to be very fast. The owner can usually grab them quite easily due to practice, and an unconscious mental effect she will have on their movements (someone with a trained eye may notice that Ioun stones' orbits will act like a mood ring, in giving away their wearer's feelings).
Ioun stones are best kept hidden in the presence of strangers, or in primitive cultures. They are very unnerving for most people to watch, and can possibly get the wearer branded as a witch, demon, or heretic, depending of course on the area's beliefs about magic in general.
These stones are usually found in groups of several stones. But just like magnets, identical stones repel each other, so you will never find two stones with the same power orbiting a single person, as one of them would simply fly away. Most groups will have a few of the useless grey stones that used to have powers (these stones do not repel any types at all, including each other). These powerless stones are often painted, and sold by charlatans at high prices, who will claim that it is a functioning stone of a different type (the powers provided by most stones are subtle enough to easily fool someone).
This is only my interpretation of this item, which is based largely upon its use in the older versions of the Dungeons & Dragons series of games. Your set of Ioun Stones may be totally different, so always be sure and read the instructions.