Rock form in a variety of geologic setting ranging from locations on or near the earth surface, deep underground, or even in outer space.
Most of the rocks we see on the surface of the planet formed by processes that happened long ago, but we can see these processes actively taking place in many places.
Rapid rock formation can be seen happening such as lava cooling from a volcanic eruption in places like Hawaii or Iceland.
However, most rocks we see around us form very slowly in settings that are not visible on the land surface.
The science of geology is founded on basic principles that are useful for making observations about the world around us.
This chapter presents a mix of information that is essential (fundamental) to all following chapters.
In contrast, minerals considered "gems" are, mostly, exceedingly rare.
A rock is a relatively hard, naturally formed mineral or petrified matter; a naturally formed aggregate of mineral matter constituting a significant part of the earth's crust.Rocks are composed of chemical compounds naturally occurring in nature.Rocks are composed of particles ranging from microscopic grains to full sized crystals and crystal grains of different kinds of minerals, and containing many different identifiable physical characteristics.Stone is another common term used to describe rock. Figure 2-2 shows how minerals can be combined to form different kinds of rocks that form under different environmental conditions.The mineral composition of a rock reflects the physical environment and geologic history where a rock formed.The chemical composition of Earth's crust has similarities with other stony planets, with silicate-rich rocks being dominant in most locations on the surface.