Looks like a coticule. Could even be a good one. The back seems to be slate, so pay no attention to it at this moment.

I would dress the "yellow" side with abrasive powder (if it is not straight, I start with 80 grit since some are quite hard). Finish with 800-1200. I have some I only use like this as final finisher, always on running water.

If you like to try it with slurry, get a diamond plate, make sure it's worn-out first (using it on a novaculite worked best for me).

Sandpaper is a no go mostly due to the fact that even the very best do give some particles to the surface at some point.

Since you are new to all of these, try finding a friend with the proper stuff and test. Not worth buying yet since you don't know if you need them or not.