Preseli Bluestone is a Dolorite rock. This is a metamorphic igneous rock, equivalent to basalt in composition, and is harder than granite. It exists in two forms, spotted and unspotted. Both forms are found at Stonehenge. It is a medium grained dark rock, with colours ranging between blue, green and grey. It may contain flecks of secondary minerals, like quartz, that the ancient tribes believed looked like the night sky.
This rare and enigmatic stone is mainly found in the Preseli mountains (hills), Pembrokeshire, Wales, where it is protected in a Site of Special Scientific Interest.