A relatively common aggregate of mineral grains. Some rocks consist essentially of but one mineral species (monomineralic, such as quartzite, composed of quartz); others consist of two or more minerals (polymineralic, such as granite, composed of quartz, feldspar, and biotite). Rock names are not given for those rare combinations of minerals that constitute ore deposits, such as quartz, pyrite, and gold. In the popular sense rock is considered also to denote a compact substance, one with some coherence; but geologically, friable volcanic ash also is a rock. A genetic classification of rocks follows:

• Igneous
• Intrusive
• Plutonic (deep)
• Hypabyssal (shallow)
• Extrusive
• Flow
• Pyroclastic (explosive)
• Sedimentary
• Clastic (mechanical or detrital)
• Chemical (crystalline or precipitated)
• Organic (biogenic)
• Metamorphic
• Cataclastic
• Contact metamorphic and pyrometasomatic
• Regional metamorphic (dynamothermal)
• Hybrid
• Metasomatic
• Migmatitic