Gneiss is a tough, hard, coarse-grained metamorphic rock. It looks like it has ribbons or stripes of different coloured minerals running through it. It is usually light in colour, but it can be quite dark.
It can look similar to granite. But in granite the crystals are randomly aligned, whereas the crystals in gneiss are lined up in layers. The light coloured crystals in gneiss are usually quartz or feldspar, the dark coloured crystals are mafic minerals.
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How was it formed?
Gneiss is formed from another metamorphic rock called schist, which itself started out life as a sedimentary rock called shale. To form a gneiss you need to subject the original rock to very great pressures and allow time for new large crystals to grow slowly. Because of the high pressure, the new crystals will tend to grow in layers or bands.
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