The island of Iona is about 1km off the south west tip of Mull. it is world famous for its religious significance as one of the earliest centres of Christianity in Britain. Among geologists it is also known for being a source of Marble from a quarry on the southern end of the island. Less well known is another outcrop on the western side. This is different from the southern marble as it contains phlogopite mica. This gives the stone from here a "sparkle" and is known as Silverstone.
http://www.geosecslides.co.uk/ It is a fine walk in good weather, from the ferry you need to allow 4 hours for the round trip, as there is plenty of interesting geology (lewisian) and to distract you, and beautiful views to photograph.