@
marsbar ah, ok.
The thing with working with abrasives and marble , is that fine particles of the abrasive can sorta etch into the stone.
And lucky you, you have two TOTALLY different colours. 😄
Maybe not try the fine steel wool on the light one just in case .
Toothpaste on a cotton cloth, or even dampened kitchen paper is a SUPERFINE abrasive....a.polish really .
If its not strong enough to get out the stain, it might help reduce the scratching or dullness you might get after using the baking soda or S.wool.
Use it as a polish after the above products.
Sometimes you have to treat stains on marble like the duco on a car ...you have to actually cut in , and polish out .
And if you need to do this....you don't want to keep doing it every time .
That's where the wax protection comes in .
There are now liquid ceramic coatings you can buy for stone that protect it .
Not cheap , but I don't know how much a hard water filter is by comparison.
Marble is sorta high maintenance, but it is beautiful.