Not sure that I fully understand the question, Guest, but slate is a pretty neutral material and can be used as a decorative top dressing around anything.

I hope so as we have a recreation of a slate waste heap made from Welsh slate with lots of iron in it, turning the slate lovely shades of red and orange.

Slate is expensive to buy. I would advise putting down a membrane around your plants before placing the slate on top, to stop them disappearing in to the soil. Owdboggy you say there is a lot of iron in the slate does that have an effect on plants. Eg herb. perennials. Or does the iron not leach out into the surrounding soil.

I would imagine that Owdboggy's iron is in the form of pyrites crystals? A great addition to the apperance.

Iron is no problem, but lead is another thing--be careful if it comes from a mine!

I cannot see that lead would be a problem in an area where no food plants are to be grown. There was certainly no shortage of plant life of all kinds growing in around and on the slate mound from which we took the pieces.

Yyyeeesss...As long as you or the future owners don't try to grow veggies downhill from there, or there at a later date! At least there are only a few places in the UK where there are lead mines.

You are joking! There are more lead mines in the UK than anywhere else in the world. just not in use any more. The Romans got almost all their lead from Britain.
I can find no warnings anywhere about Welsh slate of any description being contaminated with lead though.

I guess this is another case of my knowing just enough to be dangerous! I knew that there were a lot of lead mines in the UK, many of them near the slate deposits of Yorkshire. But I was expecting the vast majority of slate to be quite safe...which is what I should have said, instead of what I did! Oh well, sorry about the misapprehension!

Interesting. Never heard of any slate mines in Yorkshire. There are some over the border in Cumbira where there was a lot of lead mining,
The majority of the slate sold as ground cover material is the waste material from the spoil heaps, so it has been exposed to the rain for many years. Any contaminants which can be leached out will already have been.
One thing which you can find in Honiston slate from Cumbria are tiny, tiny diamonds, like dust size and very rare.

I was thinking that the spoil heaps from the mines in Colorado are still leaching after 150 years. Then I remembered that some of your mines are thousands of years old! I have to admit that my knowledge of the mines in Yorkshire comes from "James Herriot"'s book about the Yorkshire Dales, James Herriot's Yorkshire--this one isn't fictional, but I am not one hundred percent sure of my memory of it! Fascinating about the diamonds, too.

You were absolutely correct to point out that there might be a danger of lead poisoning from any material put on the soil.
The saddest thing here is that most slate now is imported from China rather than using the masses of it still left in the Welsh Hills.Heaven knows what there is in it.