Bits of flint to rough out and odd lumps of sandstone to soften it. There are cavities ground into some rocks that are not in the Rock Art catalogues yet - they are more interesting. Deep irregular holes (this shape http://www.themodernantiquarian.com/post/45353 but deep. I've some photographs - they might not be developed before Christmas ! And stones that are perforated with round holes are acceptable, yet others, with square or rectangular holes, are excluded. 'Gateposts'. Here's a stone that's lying in the open as nobody has recognised that it's carved - http://www.themodernantiquarian.com/post/44496 - and it's not 'fossils' on the surface, it's more of these odd 'cupmarks'.