Although at least 20 types of rocks have been identified at the site, Stonehenge is made from two primary types of stone: sarsen sandstone and bluestone, with the stones forming the external wall of the circle in Stonehenge being made from the former. Sarsen sandstone is around 60 million years old and is essentially silicified sandstone that is much like the sandstone that is found about 19 miles to the north of the world-famous monument.
The pie-shaped structures for which Stonehenge is so readily known for are made from sarsen sandstone. The largest of these weighs an estimated 40 tons, which is the weight of a cement truck that's fully loaded. The size of this stone and others has led to many theories about how the stones arrived at the location in the first place.
The smaller of the stones at Stonehenge are bluestones, and although they are smaller than the sarsens, some are still massive, weighing up to an estimated 4 tons. They are foreign to the area, leading to even more speculation as to how they arrived there.
There are other types of rock at the location, including gneiss, schist, limestone and greenstone, along with some unidentified sandstones. The "Altar" at Stonehenge is from a foreign sandstone of unknown origin.