i agree with Slabbercabber, you have created a VERY dangerous machine for the following reasons.
1) I don't see that the grinder is set up for water. That likely means that the electrical is not protected from water. Water + electricity is bad.
2) The grinding wheels are not designed to be used with water. One of the limitations of the old silicon carbide type wheels (and these) is that they have to be spun completely dry otherwise one part of the wheel with water is heavier than the rest of the wheel. The heavy part of the wheel comes apart explosively. I saw a video yesterday of a guy who died when a wheel exploded into his chest. To use the diamond flat disks it looks like you have to take off the wheel guards which increases your risk. When the equipment manufacturer puts a safety device on the equipment it is a good bet it is needed.
3) Using this grinding machine without water cooling (as designed) will cause your rock to heat up a lot. You cannot dunk the rock in water enough to keep it cool, plus we are back to getting the wheels wet again. If you are lucky the rock will just keep falling off of your stick. If you are a little unlucky it will crack or break up. If you are very unlucky it will shoot pieces of the rock back at you.
This equipment was not designed for rocks and cannot be made safe to grind rocks. Ameritool makes a very good simple swap top grinder at around $450. I have one and love it. Stay away from Inland Lapidary as their machines are junk (had one of those also).