Rocks, whether pebble or mountain sized, are composed of one or more minerals. Minerals are naturally occurring elements or chemical compounds with a unique crystalline structure; among these compounds are various sulfates, sulfides, halides, carbonates, nitrates, oxides, hydroxides, phosphates, silicates and others. It is the chemical structure of a mineral that determines its physical characteristics, such as hardness, transparency, friability, color, luster, specific gravity, radioactivity and fluorescence. To date, more than 3000 minerals have been identified on Earth; common examples include quartz, talc, dolomite, copper, gold, rock salt and hematite.
Rocks are broadly classified as igneous (having formed from magma), sedimentary (having formed from debris that settled in topographic basins, along stream channels or at the bottom of lakes and oceans) or metamorphic (igneous or sedimentary rocks that have been transformed by heat and pressure). Granite is the classic example of igneous rock, sandstones, shales and limestones are common sedimentary rocks and both marble and slate are well-known metamorphic rocks. Igneous rocks are sub-classified as intrusive (having cooled beneath the surface), extrusive (having cooled above the surface) or hypabyssal (having cooled within fractures or between layers of the continental crust); laccoliths, plutons and batholiths are examples of intrusive rock formations, basalt flows (the Deccan Traps, the Columbia Plateau) provide a classic example of extrusive rock strata while pegmatites, amalgams of various minerals and rocks, offer an example of hypabyssal formations.
Of course, this outline is deceivingly simple. Even minor changes in the mineral content of rocks can dramatically alter their appearance and other physical characteristics. But an amateur rockhound has to start somewhere and these general classifications provide a basic structure for delving into the complex geochemistry of our home planet.
