Some uses of igneous rock include serving as material for buildings and roads. Igneous rocks reduced to gravel size serve as ballast for railroad beds. Igneous rocks are also used for countertops, backsplashes and sinks. They can be carved into works of art and weapons.
Some igneous rocks, such as lamproite, contain gemstones such as diamonds and spinel. Pegmatite, a coarse-grained rock very much like granite, contains useful minerals and elements such as boron and lithium. Beryl and tourmaline can also be found in pegmatite.
Granite, a hard rock that's formed by magma that has slowly cooled in the Earth's crust, is sought after not only for its toughness but its beauty. It can come in colors of white, red, pale greens and blues, grays, blacks and violet. Because of this, it is prized for home decoration and is used for flooring, countertops and table tops.
Basalt is the most common igneous rock and forms much of the seabed. It's often black, but can be gray, brown, pale green or red. Ancient Egyptians used basalt as an ornamental stone.
Obsidian forms when thick, hot lava cools quickly, before the minerals have a chance to crystallize. This gives the rock a glassy texture. Prehistoric people used obsidian to make sharp points and blades.