It is an igneous intrusion created by volcanic action. The quality of its granite used for curling stones is world famous. Nowadays only loose rocks may be retrieved for that purpose. The island has large gannet and puffin colonies and to protect them visitors are discouraged from landing. In earlier times it was the haunt of pirates and the island’s small, 16th c. tower-house was believed to have been built as a lookout and deterrent for them, containing as it does a small pit prison.