Lap it and also raise a slurry and post more pics. Any identifiable sawcuts on the ends or sides? When you lap it let us know if it seems hard or soft.
II think I'm going with some type of slate. Doesn't really look much like a Thuri to me though.
I wouldn't rule out synth unless there's sawmarks. A slate hone is almost never found without sawmarks unless someone recently got a hold of it and lapped it on all sides.
8x2" and no sawmarks generally means it's an old stone that was more of a shop stone than a barbers hone. I've had some grey, gritty, sparkly stones that fell around the 2-3k range and reminded me of very hard tightly-formed carborundum which I was never able to determine if they were synth or some natural I was unfamiliar with. I always considered maybe Ohio Blue/QC.
I've had plenty of white Queer Creeks, but I've been told there are blue examples to. That would be the only natural I could venture a guess at, but I'm certainly not confident in it.
QC came in 8x2x1 as well, but they were a lot less common. The 6x2x1's are all over the place (it came in 4",3" and slipstones too, and probably some other cuts as well).
Based on accurate measurements with digital calipers and precision rule, if it started out as an 8x2, someone has lapped off a bit on all sides. An 1/8" on the ends and sides.