Those are dark gray trans stones with a structure more like a trans stone (no large foreign particles like you can get with some black stones from some vendors), and they have some translucence, which you definitely don't get with a jet black stone like the dan's stone I showed.
Those stones are actually pretty inexpensive for what they are (being a large trans). Norton sold a lot of stones that were dark gray trans, too. Their current black stones aren't like that, at least not all of them, but a lot of the vintage ones are.
Great stones. Only complaint one could have about them if trying to substitute them with a white trans stone is that sometimes easily seeing the black swarf is useful, especially if you're trying to figure out if something needs to be heat treated (steel that's not as hard as it should be will easily hone away even on a trans stone, but stone around 60-62 hardness will almost cease to be cut by a stone that is settled in).