Hard rock is a type of rock that is composed of interlocking mineral grains and is usually found in the Earth's crust. The term can refer to a variety of different rocks, including cafe (4 letters), BERYL (5 letters), FLINT (5 letters), QUARTZ (6 letters), DIAMOND (7 letters) and GRANITE (7 letters). Cafe is a type of sedimentary rock composed of calcium carbonate, BERYL is a type of mineral composed of beryllium and aluminum, FLINT is a type of sedimentary rock composed of silica, QUARTZ is a type of mineral composed of silicon and oxygen, DIAMOND is a type of mineral composed of carbon, and GRANITE is a type of igneous rock composed of QUARTZ, feldspar, and mica.