I posted in the other thread, but yes, it's granite. It has been dived. There's not that much of interest unless you can do SCUBA to be honest. Largely featureless. I have a decent hi-res scan of the abandonment plan if anyone is interested. I wouldn't visit without permission, as there is a rather large woofy thing with teeth resident there that caused me to promptly invent teleportation when walking along the road after checking out the route of the old tramway. There may be older slate working inside the property boundary somewhere (10 acres or so).
If I had half a million kicking around though, I'd have it in a heartbeat.
Searching for the ever elusive Underground Titty Bar.
DDDWH CC