Used monster few days ago, loved it. Honestly thought it'd be harsh, wasn't at all. Had a nice bite to it and worked pretty fast.You have this backwards., Onyx marble is incredibly soft and has no relation to Onyx, the gemstone, which is black. Onyx comes in different colors, which can also vary in hardness. I think white is probably the hardest, while red is the softest, the deep green is in between.
It is very dense though, and I have never seen it get stained and have never need to seal it.
"marble" has different levels of hardness, they are not all the same and can vary significantly
Brady is right, Serpentine is much harder than most marbles
It is very easy to get trailing scratches when refinishing onyx, so cleaning the surface thoroughly between gits and making sure your diamonds are clean, scrub the channels with a brush so old, hardened slurry doesn't fall out and scratch.
Polishing can be one with a number of polishes. MB-22 works great, our Monster polish works well too, it's got a very fine grain to it. Use a white pad to polish.