Bluestones

Bluestone is a cultural or commercial name for a number of dimension or building stone varieties, including:

The term "bluestone" in Britain is used in a loose sense to cover all of the "foreign" stones at Stonehenge. It is a "convenience" label rather than a geological term, since at least 20 different rock types are represented. One of the most common rocks in the assemblage is known as Preseli Spotted Dolerite—a chemically altered igneous rock containing spots or clusters of secondary minerals replacing plagioclase feldspar. It is a medium grained dark and heavy rock, harder than granite. Preseli bluestone tools, such as axes, have been discovered elsewhere within the British Isles. Many of them appear to have been made in or near Stonehenge, since there are petrographic similarities with some of the spotted dolerites there.

The bluestones at Stonehenge were placed there during the third phase of construction at Stonehenge around 2300 BC. It is assumed that there were about 80 of them originally, but this has never been proven since only 43 remain. The stones are estimated to weigh between 2 and 4 tons each. The majority of them are believed to have been brought from the Preseli Hills, about 250 kilometres (150 miles) away in Wales, either through glaciation (glacial erratic theory) or through humans organizing their transportation. If a glacier transported the stones, then it must have been the Irish Sea Glacier. In such event, one might expect to find other bluestone boulders or slabs near the Stonehenge site, but no such bluestones (apart from fragments) have been found.

The archaeological find of the Boscombe Bowmen has been cited in support of the human transport theory, while new glacier modelling supports the erratic theory. Preseli Bluestone dolerite axe heads have been found around the Preseli Hills as well, indicating that there was a population who knew how to work with the stones, but researchers reporting in 2015 found no firm evidence of quarrying at Rhosyfelin in the Preselis.

...
Wikipedia