Very simply the rocks of Unst fall into five groups. Most of the western side of the island, including the hills of Valla Field and Hermaness, and the hill of Saxa Vord consist of various schists and gneisses. These hard rocks are not permeable, so water cannot permeate through which means that the hills are covered in waterlogged peat forming blanket bog. In various places, semi-precious garnets can be found amongst these rocks, especially on beaches where the rock is exposed by the sea.
Woodwick on the west coast is one of the best places to find stones, although an expert eye is needed to spot the unpolished stones.
Through the centre of the island runs a band of serpentine. This is an unusual rock, only found in a few places in Britain. It is permeable so water does not usually stand long, although many areas become waterlogged in winter. The vegetation growing on serpentine is often unusual, with grasses often replaced by sedges to produce so-called "sedge-lawns". The vegetation is most impressive on the Keen of Hamar Nature Reserve.
The serpentine is also a useful economic asset for the island, as the softest rock, known as soap-stone is quarried and exported as talc, used as an industrial lubricant. The quarry at Quoys has now closed but an active quarry is present at Clibberswick. Also at Clibberswick is a rocky beach called Cross Geo where the visitor can find sea-washed pebbles made of the sea-green semi-precious mineral serpentenite, which runs in veins through the serpentine.
To the east of the serpentine lies a band of gabbro or greenstone, a rock which is similar to serpentine without any of its unusual qualities. In the south-east of the island is a band of slate at Muness, a soft and easily eroded rock, which in some areas has been weathered into strange forms which resemble bark or tree-trunks. In the north-east at Skaw there is an outcrop of very coarse-grained granite.