Igneous rocks are grouped by the size of their crystals and the minerals they contain. The minerals in igneous rocks are grouped into families. Some contain mostly lighter colored minerals, some have a combination of light and dark minerals, and some have mostly darker minerals. The combination of minerals is determined by the composition of the magma. Magmas that produce lighter colored minerals are higher in silica. These create rocks such as granite and rhyolite. Darker colored minerals are found in rocks such as gabbro and basalt.
There are actually more than 700 different types of igneous rocks. Diorite is extremely hard and is commonly used for art. It was used extensively by ancient civilizations for vases and other decorative art work (Figure).
This sarcophagus is housed at the Vatican Museum. The rock is the igneous extrusive rock porphyry. Porphyry has large crystals because the magma began to cool slowly, then erupted.