The Altar Stone at Stonehenge is unique, no other stone at the monument on Salisbury Plain constitutes the same lithology; a greenish sandstone thought to be of Late Silurian-Devonian (‘Old Red Sandstone’) age.
More at
https://landscapeandmonumentality.wordpress.com/2020/07/02/the-source-of-the-altar-stone/
...which summarises the paper by Bevins, Ixer et al:
Constraining the provenance of the Stonehenge ‘Altar Stone’: Evidence from automated mineralogy and U–Pb zircon age dating by Richard E. Bevins, Duncan Pirrie, Rob A. Ixer, Hugh O’Brien, Mike Parker Pearson, Matthew R. Power, Robin K. Shail.
Journal of Archaeological Science 120 (2020)
https://www.sciencedirect.com/science/article/abs/pii/S0305440320301102 or https://doi.org/10.1016/j.jas.2020.105188
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