The web is a strange place. Here's a mad dentist link:
http://www.groupsrv.com/science/viewtopic.php?p=213518
Which should take you to:
http://today.uci.edu/news/release_detail.asp?key=1123

Non-existant babyteethmegaliths and aside, it's got to be worth a shot. I know millstone grit has a fair bit of quartz in it, and most carvings are on sandstones. If enough R&D was put in, some boffin somewhere should be able to do it.

I guess you'd need a motif that was as fresh as possible, so that the pecked surface wouldn't be as erded as the unpecked surface, as equivalently eroded surfaces on more weathered panels would give the same measurement (I think)

I'd never heard of QHD 'till then, cheers for that one FTC! :)