Q: I have a large piece of black slate, ½ inch thick, that I’d like to use for a table top. What is the best way to cut, polish, and seal it.?
A: Slate is fairly soft as stones go. A masonry blade in a portable circular saw will cut it. Diamond blades are now available for these saws that will cut any stone without needing water to protect the blade from overheating, but make sure you use a good-quality dust mask if you are doing this, since stone dust is bad for the lungs and dry cutting makes a lot of it. It is amazing how quickly a little stone-cutting will fill an indoor space with dust—if you must do this indoors, isolate the area you are working in with plastic sheeting to minimize clean-up.
I find it helpful, when cutting stone slabs, to use a plywood straight-edge with a guide attached for the saw base to run against. The saw glides on top of the straight-edge, which stops just short of the blade. This prevents the saw base from scratching the stone, as well as guiding the cut. Slate doesn’t take a polish well, and is too soft to hold a polish for long in this use, but it can be sealed with various proprietary organic sealers. Check your local tile supply for a selection.