Removing stains from Slate

[Aprisa]

I bought the above Slate Hearth and stupidly did not clean it properly before fitting.

I now see that it has the marks shown in the photo, I can’t work out what they are exactly and although it looks like paint or cement and they appear slightly proud they don't seem to change when scraped with a blade.

Have you got any suggestions as to the most caustic cleaner I can try without damaging the slate itself, or any other methods that may clean it up?

Thanks

[Aprisa]

Sorry photo did not work

[JoshSm]

When I've seen that previously it was part of the stone. Potentially it's part of a layer that you could remove but not an easy thing. I just treated it as a feature of natural stone.

[Aprisa]

Cheers Josh, initially I thought as you do but they seem raised, would that be because when slate is split then anything in that layer that is a slightly different material could appear raised?