Patios, walkways, water features, walls, wall caps. Flagstone is a sedimentary rock that is split into layers along bedding planes. Flagstone is usually a form of a sandstone composed of feldspar and quartz and is arenaceous in grain size (.16mm – 2mm in diameter). The material that binds flagstone is usually composed of silica, calcite, or iron oxide. The color of the rock usually comes from these cementing materials. Typical colors of flagstone are red, blue, and buff, though exotic colors, exist.