Petrology is the branch of geology that studies the formation, composition, and classification of igneous, sedimentary, and metamorphic rocks. Rocks adapt to changing environmental conditions by the forces that enact upon them. Rocks that are weathered by water and erosion are quite different in look and mineral composition than rocks cured in the fires of volcanic magma.
There are three basic classifications of rocks: igneous, metamorphic, and sedimentary. Sedimentary rocks are simply rocks made of sediment. Igneous is also known as "fire rock," while metamorphic rock is rock that changes or "metamorphoses" from either sedimentary or igneous. They differ in mineral composition, size, and shape. They also differ in how they form, as depicted in the "rock cycle," which explains the interrelationship among rocks and how they change through geologic time. The rock cycle is often depicted in pictorial or animated form.
Geologic time is different than time as humankind understands it. Civilization runs on a 24-hour clock, which can be thought of as a cycle of day and night, sun and moon, or a combination of days, months, or years. Geologic time operates on a much grander scale: the earth's formation over millions of years--although there is a growing number of scholars who dispute the age of the earth, it is generally accepted that the earth is around 4.6 billion years old.
Igneous rocks are the oldest rocks on earth, formed by the cooling of the fiery magma that helped create it. As oceans formed, they helped drive the organic and inorganic sediment, swept by the ocean's current, settling it into layers which ultimately were crushed into sedimentary rock as eras passed. This is easily seen in the layering of the Grand Canyon. When you look at the layers of rock in the canyon, you are seeing the history of the world all at once. Limestone is a common sedimentary rock and the oft-beautiful, glassy, smooth black obsidian is an example of an igneous rock.
The cycle continues as metamorphic rock evolves from intense pressure and heat from plate tectonics in the earth's crust. If you look at the rock cycle, igneous and sedimentary rocks often evolve into metamorphic rocks. Marble is a common metamorphic rock and is most sought after for building material. Feldspar and quartz are often considered metamorphic rock, but are actually light minerals. Silver enthusiasts still pan for gold and silver in lakes in rivers hoping to find that elusive, coveted nugget that silver jewelry and gold medallions were made from in days of yesteryear. Today, most people pan for fun with such "pans" as colanders.
The rock cycle would likely not be in existence without Scotland's James Hutton, the 18th century founder of modern geology, best known for his seminal work, Theory of the Earth, which many consider the basic text for modern geology. He defied the convention of Biblical scholars at the time who maintained that the Earth was no more than six thousand years old. Hutton wrote that the earth was in constant evolutionary phase, appreciably older than otherwise believed. This change is known as uniformitarianism. Scientists today are leading expeditions through the Grand Canyon to back up the original theory of the Biblical scholars as the debate continues to rage through the scientific world.