I haven't really seen them, but you talking about 4cm wide "stones" got me thinking of LECA (Light Expanded Clay Aggregate). If that is the case, they are very light, more like pumice stone than like real stones, so not very heavy, probably not more than normal, smaller gravel stones.
I can't confirm that the gravel traps are indeed filled with LECA, but googling LECA and montmelo returns a bunch of hits about gravel traps in the circuit, but somehow only in spanish and only from not authoritative sources.
Edit: found it. It is probably LECA.http://www.netweber.co.uk/uploads/media ... ars_wv.pdf
Windmills are cool, elegant and magnificent. TANSTAAFL!