Arran lies in the Firth of Clyde, clearly visible from the Ayrshire coast, the rocky peak of Goatfell, its highest point, frequently capped, even on sunny days by persistent clouds. The island has highly contrasting and unusual rock formations and has become a destination for generations of geology students, also drawn by its beautiful scenery and mild climate.

The north part of the island comprises steep sided granite mountains which are the remnants of a Tertiary age volcano linked by deeply eroded and knife like ridges and arêtes. No agates have been found here but quartz in the form of “Cairngorm” or smoky quartz can be found at a number of localities on Goatfell and the surrounding hills.

Agates of Old Red Sandstone age occur in secondary conglomerate deposits along the east coast of Arran south of Brodick. It is said that an agate horizon passes through Amchrie and Corrygills and becomes exposed on the associated beaches. Most Arran agates are small but can be found in bright colours and patterns exhibiting the “fortification” pattern. Clyde coast agatised corals are also occasional finds on southern Arran particularly at the beach near Kildonan.