Dolerite from Carn Goedog (Preseli, Wales)
Stonehenge building blocks quarry
 

Spotted Dolerite "Bluestone"
Mafic, holocrystalline, subvolcanic rock
Preseli, Wales (UK)
Specimen is a 3.12g slice

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 The Preseli Hills or, as they are known locally and historically, Preseli Mountains (Welsh: Mynyddoedd y Preseli) is a range of hills in north Pembrokeshire, west Wales.

The "Preseli Bluestone" is a form of Spotted Dolerite (a plutonic igneous rock, equivalent to basalt in composition, composed of one- to two-thirds feldspar with a silica content of less than 55%, of which quartz forms only 10%).

Of the 43 identified Welsh bluestones at Stonehenge, 30 are of dolerite (mostly spotted). These have long been recognised as coming from Preseli (Thomas 1923). He (H.H. Thomas) recognised the distinctive ‘spotted dolerite’ from his fieldwork in that area and suggested that the tors of Carn Meini (also known as Carn Menyn) and Cerrigmarchogion were the most likely sources. In the early 1990s, in a major contribution to the understanding of the Stonehenge bluestones origin, the geochemistry of a set of samples from Stonehenge monoliths and debitage was determined and compared against the geochemistry of dolerites from the eastern Mynydd Preseli by a team from the Open University led by R.S.Thorpe (mainly, but not only, the compared concentrations of the so-called ‘immobile’ elements - elements which are not affected by rock alteration processes), in particular TiO2, Y, and Zr.

In 2014 additional works found Carn Goedog (still in Preseli Hills) to be the best match for most of the Stonehenge Bluestones.