Granite and Gneiss are two of the most common types of igneous rocks.Granite is a coarse-grained intrusive igneous rock with a medium to high level of silica. It typically has quartz grains, plagioclase feldspar, hornblende, biotite, and muscovite. With proportions roughly equal of plagioclase and potassium feldspar, the granite is considered felsic in composition. Granite is found in many parts of the world, from the Earth’s core to the surface. It is typically gray or pink in color, and has a distinctive coarse-grained structure.
Gneiss is also a medium to coarse-grained intrusive igneous rock with a high level of silica. It is made up of quartz grains as well as schist minerals such as biotite, muscovite, and garnet. Gneiss is usually composed of alternating bands of light and dark minerals. It can range from yellow to gray or pink in color, depending on its mineral composition. Gneiss is a metamorphic rock, which means that it has been changed by heat and pressure.
Unlike granite and gneiss, aplite is a very fine-grained, light-colored, intrusive igneous rock. It is composed primarily of quartz and feldspar minerals, which give it a light color. Aplite often appears almost white or tinted pink or orange.
In contrast to these extremely common intrusive igneous rocks, aphanitic or extrusive rocks such as andesite, dacite, and obsidian are much less common. Andesite is a dark-colored, extrusive rock made up of silica, plagioclase, and biotite minerals with a very fine-grained, aphanitic texture. It often appears gray, black, or brown in color. Dacite is similar in composition to andesite but its grain size is coarser and its color ranges from gray to pink. Obsidian is an extrusive black rock composed of very fine-grained volcanic glass.
In addition to these common types of intrusive and extrusive igneous rocks, there are also a variety of other types. Pumice is a type of very light-weight rock made up of volcanic glass. It is so light that it will float on water. Andesitic breccia is a type of rock composed of broken pieces of andesite cemented together. Lapilli is the term for fragments of volcanic rock the size of peas or larger.
Lastly, tuff is the type of rock made up of fragments of volcanic ash. It can range from light gray to pink or yellow in color and can vary in grain size from very fine to coarse.
No matter what type of igneous rock you are looking for, you are sure to find it or have it made. From granite to obsidian, the options are endless and the beauty is timeless.
