One intuitively chosen tumble. 
Lewisian gneiss come from the Isles of Lewis, Harris and North Highlands. They are collected from the ancient bedrock of Scotland and are one of the oldest rocks in the world, up to 3 billion years old (3,000,000,000 years) It is a unique composition of many minerals including Feldspar, Epidote and Quartz, all of which are part of the "Lewisian Gneiss" complex. Lewisian has been treasured for thousands of years by the clans of Scotland. It was used by the Megalithic cultures to construct monuments such as the Callanish stone circle on the Isle of Lewis.