Quartz(石英), which can split light into a spectrum, is one of the most common of all rock-forming minerals and one of the most important constituents of the earth's crust(地殻). Quartz may be transparent, translucent(半透明の), or opaque(不透明な); it may be colorless or colored. Varieties are classified as crystalline(結晶構造の)and cryptocrystalline(隠微晶質の), whose crystal structure can be seen only under the microscope, if at all(たとえあったとしても).