You should have no issues polishing rocks on the Tormek. Being a wet grinder it would probably be ideal. As for hardness the four wheel types are standard SG-250 which is Aluminium Oxide, black SB-250 which is Silicon Carbide, Diamond Dx-250 where the x is C-coarse, F-fine and E-extra fine, and lastly the SJ-250 Japanese which I *think* is Aluminium Oxode, a finer grade (more pure), hence the white colour, than the SG. The Hardnessess are Aluminium Oxide - depending on its purity and origin from 3-9, Silicon Carbide 9 and Diamond 10.
As I understand it most gem grinding is done using silicon carbide sine it tends to be harder than aluminium oxide and harder than most other rocks. Depending on the finish you need you may wish to get the diamond stones which an be more agressive but will take you up to 1200 grit. You probably don't want the Japanese stone since that will be too soft so after that you will either need to go to third party wheels or hand burnish.
As I understand it most gem grinding is done using silicon carbide sine it tends to be harder than aluminium oxide and harder than most other rocks. Depending on the finish you need you may wish to get the diamond stones which an be more agressive but will take you up to 1200 grit. You probably don't want the Japanese stone since that will be too soft so after that you will either need to go to third party wheels or hand burnish.