Igneous rocks are classified by examining their texture and mineral composition. They are made up of two or more materials. Igneous rocks do not have fossils embedded within their makeup nor will you see layering that is commonly found in other types of rock. Igneous rocks do not usually react to acid. They have a wide range of colors and different sizes of crystals that can be seen with or without a microscope, depending on the size of the crystals formed.
When magma flows into cracks and crevices and cools slowly over thousands of years, elements combine together underneath the Earth's surface, and intrusive igneous rocks form. Given enough space, the crystals formed can become large enough for the naked eye to see without a microscope.
When the magma erupts from a volcano or through cracks in the Earth's crust, it is called lava. The lava cools quickly, and extrusive igneous rocks are formed. These rocks have crystals, but they are much smaller than those in the intrusive type of igneous rock. Extrusive igneous rocks can have the appearance of black shiny glass due to how fast the lava cools when it reaches the surface of the Earth.
Commonly found intrusive igneous rocks are granite and gabbro. Granite is the most common of the igneous rocks and is found in many colors and hues. Granite is found and used all over the world in items we see every day. Gabbro is primarily black and dark green in color and found mostly in the oceanic crust. Extrusive igneous rocks most commonly encountered are basalt and obsidian. Basalt is similar to gabbro but with a finer grain. It is one of Earth's most common rocks and can also be found on the moon. Obsidian looks glassy and is typically dark brown, green or black in color. Obsidian is found only in parts of the world with recent volcanic activity.