Well just to add to this, there is a fascinating article in Britarch this month, 'Animating Kilmartin', some dating as well would you believe it.
Firstly, Torbhlaren, named as 'Tiger Rock' and secondly the rock panel at Ormaig which is given the name 'Lion Rock' (a good looking rocky outcrop).

It is a long article but the mention of using quartz to incise the rock art is worthy of a mention, shattered and unshattered quartz surrounded the panels, the shattered is thought to be that which broke when used, (according to experimentation by one of the archaeologists).

Sals photo on this page shows a standing stone by the rock art stone - Tiger Rock.

Aaron Watson is an artist and archaeologist, one of the three writing the article...
http://www.monumental.uk.com/