Your specimen could be either gneiss or schist. Gneiss is
formed during very high grade metamorphism, when temperatures and pressures are
extremely high, and it is thought that the parent rock (the original rock being
exposed to metamorphism) may almost melt and then recrystallise into light and
dark mineral bands. The light bands are made up of feldspar and quartz, and the
dark bands of biotite and hornblende minerals. Gneisses form from a range of
igneous and sedimentary parent rocks, and although not found in the Potteries
area they can be found in Scotland.
Schists are formed during medium grade metamorphism: the pressures and
temperatures are not as high as those needed to form gneiss but are higher than
those needed to form slate. Schists contain a range of different minerals (which
often indicate the precise condition in which the rock formed) and mica crystals
in the rock often align to produce a texture known as schistosity.
If you want to see some examples of metamorphic rocks from The Potteries Museum
& Art Gallery's collections, click here to search the Virtual Store.