In my area you are much more likely to find a 9" or thicker granite inspection surface scrapped out than a tombstone. Many auto factories and machine shops have closed and left these "stones" without a home. I measure on a 7'x4' one daily certified to plus or minus .00005 flat. If you think that granite plate is guaranteed not to warp you are mistaken, they even have to be resurfaced every year for settling and wear.
Use something else. Granite doesn't take pins well. A dollup of glue throws it way out. Balsa dust even takes it out of spec.
Find the flattest most usable surface you can and have fun. Thats what this hobby is all about.