A trans stone that passes light will be a good stone. Dan's black stones are about the same specific gravity and fineness as the best trans stones. Most other black stones aren't as dense and therefore aren't as fine.

Old Norton or pike (rare) black stones are good, if someone is pushing the old halls stock, those are ok, but no black stone that I've tried- new or old - is better than dans black.

If a full thickness stone is too expensive, get half thickness or combo to cut the cost.

As mentioned above, specific gravity determines fineness far better than anything else. The subpar black stones lack density. The good ones are mid 2.6s. At about 2.6, you start seeing pores, and pores are aggressive cutters until a stone is very broken in. That's bad.