Xuanwuyan (basalt), mainly composed of oceanic crust, belongs to basic volcanic rocks. It is the most important component of the earth's oceanic crust and lunar sea, and also the important component of the earth's continental crust and lunar land. Basalt is a basic extrusive rock. Its chemical composition is similar to gabbro. The content of SiO₂ varies from 45% to 52%, the content of K₂O + Na₂O is slightly higher than that of intrusive rock, and the content of CaO, Fe₂O₃ + FeO and MgO is slightly lower than that of intrusive rock. Minerals are mainly composed of basic feldspar and pyroxene. Secondary minerals are olivine, hornblende and biotite. Rocks are dark, generally black, sometimes grey-green and dark purple. A porphyritic structure. Stomatal structure and almond structure are common. The volume density of the basalt is 2.8-3.3 g/cm³, and the compressive strength of the compacted basalt is very high, up to 300 MPa, sometimes higher. The strength decreases with the presence of vitreous and stomata. The basalt is highly durable and jointed, and the joints are hexagonal. Because of its brittleness, it is difficult to extract large blocks of stone. Because of the common stomatal and almond structures, although basalt is widely distributed on the surface, it can not be used as a decorative stone.