I've got something somewhere about this deliberately chosen different coloured stones thing. I'll have a look.
They had found iron pyrite at Priddy henges, which is "fools gold", and I noticed in a book few months back that you can also find ammonite fossils(Stoney Littleton maybe) in fools gold. See pyrite here..... http://www.wiltshireheritage.org.uk/galleries/index.php?Action=4&obID=154&prevID=97&oprevID=16