All of it!
Eye protection would probably be at the top of my list. But seriously, use whatever you are supposed to use. If you are sanding, use the dust recovery system, and probably a mask as well. There are different masks for different purposes, make sure to find the right one. Don't take short-cuts. Don't take chances. If that is your skill and trade, you don't want to put yourself out of work by getting injured!
People who have a lot of experience. In tile and marble setting, you're dealing with a craft that is honed over decades, and very little of it is automated. Also, the end result is often only judge able by subjective approaches. IOW, it "just looks right", and explaining why requires a lot of words. Someone who gets an artistic instinct about the result as well as the various techniques and conditions that affect the final outcome is the person is very valuable to that trade and to the customer.