It is said that the ancient Greeks created the idea of 'worry stones', as a means of calming the mind. The simple repetitive motion of rubbing a smooth surface is said to bring tranquility.
Designating some physical object with the imagined power to absorb anxiety and other negative feelings, leaving you calmer as your worries melt away, is a recognised therapeutic technique. It helps to deal with uncomfortable, stressful and emotional feelings or situations, and can be helpful for adults as well as children.
A worry stone does not have to be a stone at all but my Swirl-StoNZ fit nicely in the palm of your hand and feel good to hold; they are pretty to look at and light enough to carry in your pocket or bag, so you can always have one close to hand. I prefer to call them 'calming stones' as this emphasises their positive nature.