Gneiss is a metamorphic rock often formed from high grade regional metamorphism, usually from pre-existing rock (known as the country rock). As the metamorphic processes are high grade, the original rock could have either been sedimentary or igneous. One of the most identifiable features of gneiss are its alternating dark and light foliations, known as banding, which are known as gneissic banding and composed of various dark and light coloured minerals. The dark bands are often comprised of mafic minerals such as iron and pyroxenes where as the lighter coloured bands are felsic in chemical composition, often comprised of potassium, aluminium or silicate minerals. These particular gneiss rock samples are of the famous Lewisian gneiss from Scotland, they are believed to have been igneous in origin, determined due to the rock being much more darker than other gneisses we stock, such as the Norwegian variety. It forms part of the Lewisian complex, a collective of various Pre-Cambrian rocks, and among the oldest rocks in the United Kingdom. The gneiss, and much of the Lewisian rocks were at some point caught up in the Caledonian orogeny which occured between the Ordovician to Devonian periods approximately 490–390 million years ago, which pushed most of these Archaeic rocks upwards, as outcrops on the surface. They are available in various optional sizes and come supplied in a card tray with an information label.