In reply to sflint:
There's marble around, but I'm not aware of any climbing on it.
As suggested, there is contact metamorphism of the Durness limestone (e.g. at Ledmore, or on Skye).
Other limestones in the Dalradian have been metamorphosed to marble (e.g. by Ballachulish, by the granite).
The other place to look is in the Lewisian rocks of the North West and the Western Isles - in places, carbonates have been metamorphosed to marble pods amongst the gneiss, e.g. Iona marble quarry. You could maybe boulder there, but the marble is mostly quarried out.