This stone is otherwise known as Clach a’ Druidh. It’s a massive lumpy glacial erratic boulder, over six metres long and broad, which rises four metres out of the ground. Obviously it’s the rock of the druids. Slightly less convincing is to link the name with Roderick Hael, a king of Strathclyde.
This page from the Geological Conservation Review says it is composed of a basalt, criss-crossed by haematite veins. A quick websearch suggests it’s currently (and probably always has been) the haunt of rock climbers. If you can’t get to the top, BRAC has some photos of some cupmarks that are up there.