David Weaver
Relatively plain black looking stone. one side profile showing a sparkly bit. They're subtle, but they're there.

I have another razor hone that sparkles like this, but it's small and it's got a bit of tooth like this one. No clue if they're the same stone.

Labeled lack sedimentary stones of note (water of ayr labeled or scotch dual hones) are prohibitively expensive, so I haven't had any. Some of the razor types (there's a french type called la lune) that aren't English have collectible value but aren't that fine. The la lune will sometimes say things like "only for good razors" on the stone. They leave stray scratches compared to more widely seen natural finishers (eschers, idwalls, coticules) - I would guess they need a tough razor edge to not have significant tooth (nobody wants "tooth" when shaving).

I think these sedimentary stones work fast on water, fast on oil - if they autoslurry as this one does. But you can always make them cut slowly by using them with oil and them using water as the lubricant. They almost cease to cut and leave a very bright polish. I don't think it's a practical method, though.