On a similar note. I recently found a rock in the area above Cwmorthin where it looked as if untopping activity had been going on.
It is a light whitish grey with straight dark and lighter fine grey lines running through it.
Sorry I haven't got a photo of it yet but was wondering if given the location where I found it, someone could hazard a guess.
By the way, I don't advocate large scale mineral and stone collecting, which leads me to wonder just how many of the unusual stones in the articles above have been collected from their Gobi desert environment. 😞
'I wonder how many breakfasts, and other meals we have missed inside that nasty clockless, timeless hole?'
'The Hobbit'
J R R Tolkien.