As a child I recall digging through my yard, examining all kinds of things; bugs, dirt and, of course, rocks. I found all kinds too. I came across big ones, small ones, dark ones, shiny ones, etc. Once I reached High School, I finally found out even more about the pastime I loved as a child. The most basic lesson was that all rocks are categorized into three specific groups; Igneous, Sedimentary, and Metamorphic.
The Earth's surface is composed of igneous rocks because it is exposed to the cool atmosphere. They are essentially crystalline solids which form from the cooling of hot, liquid magma. There also various forms of igneous rock. You can further classify them rock by how it feels (texture) and what it's made of (composition).
When a volcano erupts, magma spills out onto the earth's surface when it cools and becomes solid. These rocks are fine grained and called "extrusive". However, sometimes magma does not come to the surface. Instead, it forces its way into cracks and spaces between other rocks already inside the Earth and turns solid before ever reaching the surface. These slowly become solid because it takes it a long time to cool inside the hot Earth. These rocks become course and are called "intrusive".
The other factor is composition. The elements in the magma directly affect which minerals are formed when the magma cools. You basically have felsic, intermediate, mafic, or ultramafic.
Felsic rocks contain the lighter elements such as silica, oxygen, aluminium, sodium, and potassium. They are usually light in color and have specific gravities less than 3. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium rich plagioclase feldspars. The most common felsic rock is granite. Mafic rocks are darker and have a high specific gravity. The other categories, with the exception of ultramafic (because the conditions needed to produce this type do not exist in nature at this time) lie in between these two opposites.
Sedimentary rocks are formed from loose sediment usually found on top of igneous rocks. The layered rock result from this debris get stuck together. Sedimentary rocks are often the result of the accumulation of small pieces broken off of pre-existing rocks. There are also three more categories in which to further sort this group.
Clastic: These basically look like clumps of rock stuck together.
Chemical: These usually form when standing water evaporates, leaving dissolved minerals in its place. Salt is a usual outcome.
Organic: Organic rocks are the accumulation of calcium deposits such as bones and teeth.
Metaphoric rocks can be any rock. This is because rocks can change over time, due to a change in its environment. Common metamorphic rocks include slate, schist, gneiss, and marble.
Now you too know the joys of rocks. Who knew they could be so interesting? The processes of even the simplest of Earth's treasures go through so much, it is hard to view them as an ordinary object once you learn their secrets.