The vast majority of the established crags are on these sandstone cliffs, although there are also a couple of scattered igneous outcrops in the mountains (the granite at Tarana, for example).
The Blue Mountains sandstone is quite variable in quality, with everything from unclimbable choss through to rock that is as good as that found at Mt Arapiles. This variability has resulted in the numerous features found in the mountains; there's everything from enormous juggy rooves to thin crimpy slabs to exposed aretes to superb crack lines.