This is not one particular kind of stone, but usually any crystal that is sliced into a thin piece and often have a groove carved into the stone for the thumb. Worry stones are great to use in a stressful situation. You simply rub the flat stone with thumb and forefinger. It is said that the stone will absorb the stress from your body the more you gently rub the stone. Stones that are easily cut, such as agate, onyx, and quartz, make great worry stones.