From our research it appears there are at least 4 Clach's in Scotland, 3 being in Aberdeenshire. It is believed the Stone near Eassgach , Brown Cow Hill and the Vannich is the one referenced in the books.  

4 Identified locations, 3 Aberdeenshire, 1 in..

1. Near Brown Cow Hill – a Stone on a hill, see Tab titled 'Introduction'
2. Near Ben Avon – a Stone on a spur on a mountain
3. Near Tornahaish – a Stone on a Hill -  also has an interesting coincidence in elevation and contours with Brown Cow Hill
4. Near Argyle in... – a Stone on a Hill, or mountain

a.   Near Brown Cow Hill – a Stone on a hill,

A summit of W Aberdeenshire, Clach Choutsaich rises to 718m (2355 feet) on the northern flank of Brown Cow Hill, 3 miles (5 km) west southwest of Corgarff Castle.

http://www.scottish-places.info/features/featurefirst43881.html