Many lapidary polishers use recirculating water systems. These are grinders and polishers that spray water from a pan onto the wheels. These systems save a lot of time and money when compared to the type you have to run water to or have buckets with tubes hanging over your head.
There is a drawback to the recirculating systems. When you are using the grinding wheels there are little bits and pieces of grit that come off and end up floating in the water. This would be ok but the grit that has settled then gets pumped back up and spit onto the next wheel leaving scratches in your stone. This is a serious problem with silicon carbide wheels, which is why most of these kinds of machines require a fresh water supply. Diamond machines tend to use recirculating water. This is because they do not suffer from this issue as much as silicon carbide machines.
The only real solution when using diamond machines that use recirculating water is to wash the tray out after using each grinding wheel or when you notice new scratches develop in your stone as you go through he process. It is more rare for diamond wheels to suffer from this but occasionally they do.If you are looking at getting a grinding unit, keep this in mind.