Now here's an interesting thing. I was hunting about on the web (as one does) and I came across the web site for the Hunterian Museum in Glasgow, where certain slides relating to the bluestones of Stonehenge are found. These slides come from the Heddle Collection.
There are many thin sections which can be examined by geologists and the uninitiated as well. Then there are five further slides flagged up as being of special historic interest. They include two dolerite samples, two sandstones (one from a sarsen stone and another from the Altar Stone) -- and then one labelled as follows:
volcanic breccia?; showing strong foliation, with albite and/or quartz?, chlorite, epidote, euhedral zircon, and large metabasalt clastTS only
Locality: Stonehenge, [Wiltshire], [England]
Context: "Altar Stone", [Neolithic]
Catalogue number: GLAHM 148302
Very strange indeed! You can see a photo of this one on the web site. A volcanic breccia labelled as "Altar Stone"? Could this be a case of a sample being mislabelled? That would be very strange, since the labelling of all the other samples seems to be fastidious. Alternatively, is there really another Altar Stone hidden somewhere beneath those fallen sarsens?
Further info here: