From the British Museum:

Mesopotamian cylinder seals are small cylinders, generally made of stone and pierced through from end to end so that they could be worn on a string or pin. The surface of the cylinder was carved in intaglio (cut into the stone) with a design, so that when rolled on clay the cylinder would leave a continuous impression of the design, reversed and in relief. Cylinder seals were invented around 3500 BC in southern Mesopotamia (now Iraq) or south-western Iran, and were used as an administrative tool, as jewellery and as magical amulets until around 300 BC.

Image sources:  1, 2, 3, 4, 5, 6, 7.

The Morgan has a great collection of cylinder seals. They are worth seeing in person, because they are so fantastically small.