The most common igneous rock found at the earth’s surface is granite. It is widely used in construction, granite contains crystals that are visible to the naked eye due to the very slow crystallization below the surface. Granite occurs in a range of vibrant colours. Pumice is a light coloured and lightweight igneous rock ejected from a volcano at very high heat and pressure. Gases escape as the rock hardens, leaving bubbles and holes throughout the rock, and resulting in its light weight. Crushed pumice is often added to soaps to clean and exfoliate.
One example of extrusive igneous rock – igneous rock that cools above the surface of the ground rather than beneath – is obsidian. Obsidian is smooth like glass and usually jet black, although it can be seen in a variety of colors. Pegmatite is a crystalline igneous rock that forms below the Earth’s surface. Rhyolite is another silica-heavy example of igneous rock, prone to explosion in eruptions.