Rocks formed from magma deep inside the Earth are called igneous rocks. Igneous rocks can form inside the Earth, known as intrusive, or outside the Earth after being exposed to water and air, called extrusive. Extrusive rocks have little to no time to crystallize and, as a result, the crystals are very small or microscopic.
Basalt is a very dark colored extrusive igneous rock. Basalt is very abundant beneath the ocean floor and is the most common rock in the Earth's crust. It forms when the upper mantle melts. Hot spots around the world also form basalt, such as the Galápagos and Hawaiian islands which are essentially shield volcanoes.
Obsidian, also known as volcanic glass, is mainly silicon dioxide formed when magma comes into contact with water. It ranges in color from black to deep greens and purples. Obsidian might also capture air bubbles while forming, which produces colorful effects ranging from streaks to blotches.
Andesite is named for the Andes mountains and forms at continental margins at the ocean's tectonic plates. It is composed of plagioclase, pyroxene, magnetite, quartz and sphene. It may be white, gray or shades of white or gray.
Dacite is an extrusive igneous rock rich in iron and first discovered in Dacia, which was a providence of the Roman Empire, where it gets its name. It is light colored.
Komatite is a very rare extrusive igneous rock only formed from incredibly hot magma. Since the lava is so hot, it appears to be similar to water. The Earth does not have conditions suitable to form komatite and has not been in the state to form it for over 2 billion years, making any komatite formations a minimum of 2 billion years old. It appears in various shades of gray.
Pumice is light to dark gray and forms from lava that is rich in gases or air. When the lava forms a frothy texture, pumice is created. Pumice is so light and airy that many samples float on water. Pumice's rough texture makes it ideal in the beauty industry to scrub away dead and dry skin.
Rhyolite is available in white to gray shades and is a very common rock. It is similar in composition to granite, although it is extrusive and granite is intrusive. Rhyolite's crystals are very small, making them hard if not impossible to see. Rhyolite is commonly used in decorations and jewelry due to the integrated banding colors.
Scoria is dark red to black in color. It is less viscous than pumice, but forms from lava that is rich in gases. It is heavier than pumice and does not float on water. Scoria is the primary rock for cinder cone volcanoes. It does not have many uses, and its name derives from a similar term that means "waste."