Gneiss and sphalerite breccia. A brecciated rock is one that is composed of broken fragments of minerals or rock cemented together by a fine-grained matrix. This sample is composed of gneiss (grey) and sphalerite (dark) fragments. Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks. Sphalerite is the chief ore of zinc. It consists largely of zinc sulphide in crystalline form but almost always contains variable amounts of iron.