Ailsa Craig is an island in the outer Firth of Clyde, Scotland
From the mid-19th to mid-20th centuries, the island was quarried for its rare type of micro-granite with riebeckite (known as "Ailsite") which is used to make curling stones. As of 2004, 60 to 70% of all curling stones in use were made from granite from the island. The floor of the Chapel of the Thistle in St Giles Cathedral, Edinburgh is also made of this rock.
An alternative name is Paddys milestone as it lies midway between Belfast and Glasgow.