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?

Dunno. Are you thinking about colour, durability, or shape? Sometimes
people use hopelessly wrong-colour stone in gardens: just doesn't tone
in with the local stone or soil (and it's usually slate chips!).
Squared-off stones would only look good in one of those very formal
Islamic or Italian watercourses which wasn't trying to look natural, but
broken slabs could be used either way. It's tough enough for paving, so
it won't wear out.

--
Mike.