Quartz, 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. Quartz may be transparent, translucent, or opaque; it may be colorless or colored. Varieties are classified as crystalline and cryptocrystalline, whose crystal structure can be seen only under the microscope, if at all. What is quartz’s rank on the Mohs scale of hardness? Discuss
Source: The Free Dictionary