These stones are the result of a project by Austin Lockwood of the Ravensbourne Geological Society in 2000. On a visit to Lochinver, Sutherland, Scotland, Lockwood encountered a large number of blocks of gneiss quarried for sea defence structures. He arranged for 20 of these to be brought to Bromley to celebrate the millennium along with a hand specimen sized piece for every school child in the borough. There are 19 stones installed; a 20th was intended to be located at Charles Darwin's home at Down House, but this was never put in place (Paul Rainey, pers. comm.). Read more here.