There are actually two stone types used. The large uprights in the outside circle and the beams bridging these outer circle uprights are of Sarsen which is a sedimentary sandstone. The inner smaller stones are called Bluestones and are geological blends of basalt and gabbro and can be described as similar to dolerite. Essentially they are igneous blue/grey rock well suited to their generic term "bluestone" i.e. Get into the exact petrology if it is really important to you.
