"Richard" wrote ...
Someone is asking me about whether Yorkshire flagstones (3" thick slabs)
will be "any good" for a waterfall "for my pond".

What should I say?

The problem might be that the rock or a constituent dissolves in the water
causing problems especially as the water will be recirculating. Yorshire
Flagstones are a sandstone rock so there should not be a problem. To check
that they don't contain chalk pour some vinegar on them and see it they
react, if you get a froth then don't use them.

--
Regards
Bob Hobden
W.of London. UK