The sedimentary rock that would be formed by the compaction and cementation of particles 1.5 centimeters in diameter is a conglomerate. Conglomerates are formed when gravel-sized particles are deposited and then cemented together by a mineral such as calcite or silica. The particles in a conglomerate can be of any composition, but they are typically composed of quartz, feldspar, or rock fragments. Conglomerates are found in a variety of environments, including riverbeds, beaches, and mountain streams.