For those who want to know the detail of these rocks on the shore at Barvas -
Most of the Outer Hebrides of Scotland have a bedrock formed from Lewisian gneissd (pronounced 'nice'). In addition to the Outer Hebrides, they form basement deposits on the Scottish mainland west of the Moine Thrust and on the islands of Coll and Tiree. These rocks are largely igneous in origin, mixed with metamorphosed marble, quartzite and mica schist with later intrusions of basaltic dikes and granite magma.
Alternatively, you can just accept that they are attractive and leave the rest to the geologists!