I'll repeat some of the stuff I put on the Misc post under Aberdeenshire as here would probably have been a better place to post it given the points now being raised.

411 known examples at the last count.
Vast majority 7cm diameter. 12 at 9-11 cm and some oval ones.
Stone used varies considerably - sandstone, quartzite, gneiss, ...
Distribution - largely Aberdeenshire, Grampians. Also other parts of Scotland. Five found outside Scotland.
Context/Date - Skara Brae finds are late Neolithic. Reports of some finds in Bronze Age contexts but none from recent well-documented excavations.