The ringstone is a distinctive type of artefact and miniature sculpture made in India during the approximate period of the Mauryan Empire (c. 322–185 BCE) and the following Sunga Empire (187–78 BCE). They are usually dated to the 3rd or 2nd centuries BCE. They are shaped like a doughnut, but with straighter sides, and flat and plain on the bottom. They are in stone, with the top side very finely carved in relief with several circular zones of decoration running around the hole in the centre. When complete, they are about 2.5 to 4 inches (64 to 102 mm) across.

Source : Wikipedia

Most examples have been found in India and Pakistan. The intricately carved stones may have had a religious purpose, or could have been moulds for the production of precious-metal jewelry.

Other scholars have suggested that they may have been ritual objects used in meditation, or even as contraceptive devices.

None of the suggested uses have been generally accepted by research scholars as a definitive explanation.