[graniteguy52]

Gary from Manchester,
I have seen some fine stone engravings done with a relatively inexpensive sand blaster ($20 to $30) from a place like Harbor Freight, a bag of silicon sand, several layers of duct tape, and Xacto blade. And of course an air compressor. And if your like me some letters/numbers to trace onto the tape before cutting them out.
Good Luck with your slate engraving
Graniteguy52