Mull is composed of extensive lavas of Tertiary age. The island consists principally of stepped remnants of the 3000ft plateau basalt lavas poured out during the opening of the North Atlantic. These basaltic lavas are about 50 million years old and as such are a lot younger than the Andesite lavas of the Old Red Sandstone of the Midland Valley of Scotland. These lavas tend to contain fewer agates than those on the mainland.

It is possible to find agates just about anywhere on the island within the lavas but most of the best localities are on the southern coast on the Ross of Mull. These localities are in a fairly remote part of the island where road access is limited or non-existent. There is no road along the south coast of the island and road access can only be achieved at Carsaig, Loch Buie and Scoor. For that reason collecting agates on Mull is dependent on hill walking experience and fitness, as even the walk in can take 3 hours. For that reason there has been less of a search for agates in these areas than for those on the mainland.

Agates can be found in situ on the Ross of Mull at Port Nan Droighean and east along the coast from there to the beaches below Aoineadh Beag and past Carsaig Arches towards Carsaig. Other localities on the west coast include Ardmeanach and on the coastline north of Loch na Keal. Nice agate specimens have even been found in a new road-cutting just south of Tobermory.

The agates are usually blue-grey and white and both fortification and onyx specimens have been found. At Port Nan Droigheann agates can be found in the lavas and specimens up to 15cm across have been found here. One classic feature of the agates from this part of the Ross of Mull is the presence of, often large, brownish Calcite crystals. These can be in the middle of the agate and represent “early formed” crystals. Another less known fact about the agates from here is that they fluoresce a yellowish-green colour under UV light, presumably due to a small amount of Calcite within the matrix.

Some of the most beautiful and intriguing agates from the Ross of Mull have a pale honey tint and are characterised by whorls consisting of fan shaped quartz crystal groups alternating with curved bands of white or grey-white chalcedony. These agates are some of the hardest to find and are the most sough after. They are classic to this area.

Similar agates are found on some of the beaches on the nearby island of Iona but as there are no agate-bearing lavas here they are thought to have come from Mull carried by moving ice during the last Ice age.