Wy wife and I went to have a look at it today, and I must say it is rather impressive. It's very well abraded and weathered -- and the sides are so smooth and moss- and lichen-covered that it's rather difficult to see what it's made of. It's certainly igneous, but it doesn't look like spotted dolerite. The rock colour is dark blue-grey, but there is a reddish tinge here and there. I reckon it weighs at least 15 tonnes. It's a strange shape too -- like a blunted three-bladed propellor or triskell. Very different from the standard rectangular shape which we see on most erratics and cromlech capstones. It has a nice little walled enclosure around it, which uis reputed to echo when you shout -- or even whisper -- when seated on the stone.