Cuprocassiterite in quartz mica schist. Cassiterite is a tin oxide mineral. It has been the main tin ore throughout history and is still the most important source of tin today. Cupro indicates that this cassiterite sample also contains an amount of copper. Mica schists are common metamorphic rocks that can be recognised by their schistosity (fine planar alignment of minerals) and the presence of micas (silicate minerals).