Figure 13.2
Crystalline and noncrystalline materials. (A) Quartz is a mineral whose crystals have an orderly arrangement of atoms. The naturally formed flat edges (or faces) of quartz crystals reflect this atomic orderliness. (B) Obsidian is not a mineral, but is instead a type of volcanic rock that is referred to as glassy because of its lack of crystals. In noncrystalline rocks such as obsidian, the atoms are arranged randomly.(A. © De Agostini Picture Library/Getty Images; B. Bruce Gervais)