The Bowls Barrow Bluestone is better displayed than when I previously wrote about it - http://www.sarsen.org/2012/03/bowls-barrow-bluestone-in-salisbury.html (Salisbury Museum and Pastscape uses the Bowls rather than the Boles spelling so I have used it here.)
UPDATE: Pictures added 8/3/16
It looks like the base of a pillar that has been broken off and the base dug up. It looks like it came from Stonehenge and the mystery of how a large Bluestone boulder came to be on Salisbury Plain a thousand years before the Stones may be a red herring. The provenance of it coming from the Boles barrow and having being incoporated into it build is very weak.
C P Green provides an extensive examination of the question at; http://www.britac.ac.uk/pubs/proc/files/92p257.pdf
Mike Parker Pearson lays out the story in his Stonehenge book: