Earth Record
Mineralogy and formation
Preseli bluestone is a type of spotted dolerite (a medium-grained basaltic rock) that forms from slowly cooling magma intrusions. The characteristic white spots are plagioclase feldspar phenocrysts that crystallized before the surrounding matrix solidified. Found only in the Preseli Hills of Wales, these stones were transported over 180 miles to Stonehenge around 2500 BCE.
a feat that demonstrates their immense significance to Neolithic people. The exact method of transportation remains one of archaeology's great mysteries.
Chemical FormulaDolerite (spotted)Crystal SystemMixedMohs Hardness6Specific Gravity2.80-3.00LusterDull to vitreousColorBlue-GrayIMA StatusrockType LocalityMynydd Preseli intrusions, Pembrokeshire, Wales, UKIMA NumberNone (rock, not IMA mineral species) Wales (Preseli Hills)
Telling it apart
Preseli bluestone is a spotted dolerite from the Preseli Hills of Pembrokeshire, Wales, famous as the source rock for the inner circle of Stonehenge. The market confusion involves generic dolerite, dyed material, and any dark speckled igneous rock sold under the bluestone name. It is a mafic igneous rock, not a mineral species, composed primarily of plagioclase feldspar and pyroxene with distinctive white feldspar spots in a dark blue gray to green gray matrix.
Hardness varies by mineral component but generally runs 5 to 6. 5. Specific gravity is higher than average rock due to mafic minerals. If the speckled dark rock is not from Preseli, calling it Preseli bluestone is false provenance. The geological and archaeological significance is what drives value, not mineral rarity.
Spotting the real thing
Preseli bluestone: spotted dolerite with white plagioclase spots in dark mafic matrix. Mohs 6-7 (tough rock). The white spots should be natural feldspar phenocrysts.
This specific rock type comes from the Preseli Hills, Wales. Archaeological significance (Stonehenge inner ring) makes provenance important. Specimens from other dolerite localities are not "Preseli bluestone."
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