I would assume a diamond saw blade designed for rock cutting to start, but I'm not sure how what kind of surface it will leave, totally smooth or something rough that needs sanding/polishing etc..
I used a cutting wheel on a powered hand grinder. I had a large "tower style" rock but the bottom was rough so I did a flat cut across bottom - it took minutes and worked great.
Yes, thank you all. A quick “powered” solution would be preferable. I was thinking of running through a 12” or so rock once through a blade but can’t find anything that wide that isn’t industrial.
The frag band saws would be able to cut through some smaller rocks easily, 6” would probably be the biggest you could do.
A 14” diamond concrete cutting saw will only cut through about 5-6” of concrete. So that would be about minimum size needed to cut through most rocks in one pass though I wouldn’t recommend that option. A 7” grinder with a 7” diamond blade would be able to cut about 1/3-1/2 way through a lot of aquarium sized rocks, then flip it to get the rest. A 4 1/2” grinder with diamond blad could get through partially and then you could chip off the stuff with a hammer, then keep attacking it with the grinder till you get a flat bottom.
I used a sawzall with a wood blade. It worked pretty well but I had to stop and wet it all down to cool it off. Started with a diamond blade but it didn't work very well.