To cut a long story to its appropriate length:
"A rhyolite boulder collected by R. S. Newall in 1924 from an excavation at Stonehenge has been pivotal to arguments concerning glacial versus human transport of the bluestones to Stonehenge. Initial studies suggested that the boulder came from north Wales, and hence was a probable glacial erratic. New petrographic and geochemical analyses however support it being from Craig Rhos‐y‐Felin in west Wales, the source of much debitage recovered from Stonehenge. Examination of the form and surface features of the boulder provides no evidence for it being erratic. Instead, it is considered to be one more piece of debitage probably derived from a broken‐up monolith."
The full paper is available here:
Lithological description and provenancing of a collection of bluestones from excavations at Stonehenge by William Hawley in 1924 with implications for the human versus ice transport debate of the monument's bluestone megaliths: DOI:10.1002/gea.21971
Richard Bevins Rob Ixer Nick Pearce James Scourse Tim Daw
The more excitable section of the Glacial Transport true believers thought it showed signs of glacial striations where it had been dragged under the ice from Wales but the marks are just a slickensided surface. The paper: "We note that identical slickensides characterise the bedrock outcrop at Craig Rhos‐y‐Felin where we can see clear evidence of lateral movement between adjacent blocks in the outcrop".
The bullet like shape was even cited as evidence of it being ice transported but luckily Dr Brian John provides a photograph showing an identical boulder still happily lying at the Craig.
To illustrate the point further I have superimposed the "Newall Boulder" from Salisbury Museum on to Dr John's photo:
It seems unlikely that a broken bit of stone was transported to Stonehenge, though less likely things have happened, so the question is when did the pillar break. I tend to think it was probably when the pillar was being re-erected and it was an accident. Or it might have been deliberate.
Do read the complete paper, but the simple message is: the boulder and the samples taken from it show no signs of being glacially transported, it is simply "one more piece of debitage probably derived from a broken‐up monolith".