Unfortunately, there are some stones that just don't photograph well. I've tried getting a readable shot of my great-grandfather's for years using film, digital, flash, no flash and all for naught. This is the best one I have and I still can't read all of it...at least, not all the Hebrew. I've never had it clear enough to have it translated:
Have you tried using a remote flash to create a raking light across the text? For difficult gravestones I have had some success using a tripod, the camera's timed shutter release and a manually triggered flash. You need to use your lowest ISO, stop the camera down and/or use a neutral density filter to give yourself a bit of a window to get the flash timing right. Low ambient light levels also help to achieve a reasonably long exposure time. Still, it can be a bit tricky watching the flashing light on the front of your camera signal the shutter release and firing the flash quickly enough to catch it while it's still open. (Obviously a remotely triggered flash would make this an easy proposition.) I really had to wonder what the early arrivals for morning service must have thought seeing the sporadic flashes coming from the churchyard with me silhouetted crouching down beside General Alexander's gravestone.
I'm still not entirely happy with the results but they are orders of magnitude superior to my ambient light and on-axis flash attempts. I think your results using this method would be even better than mine since your grandfather's granite stone still has its original smooth face more-or-less intact. Your problem would seem to stem more from the mottled light and dark grains of the granite itself which optically soften the outlines of the carving. (The carving also may have been somewhat shallow to begin with.)
General Alexander's gravestone above seems to be some sort of quartzite-sandstone or else a very granular marble whose surface has weathered (sugared) into an irregular, high-relief texture. Even with a raking light the edges of the carving are still indistinct and fuzzy.
I have read about many techniques which claim to make gravestone inscriptions more legible. Many would seem to have the potential of damaging the stone or, at the very least, making a mess in the graveyard. My inclination is to try less drastic techniques such a lighting, imaging and post-processing first. Old_tv_nut's suggestion of "painting" water into the carving to darken the inscription seems non-destructive enough but would take some time and require a good eye and a steady hand.
If there's a certain time of day where the sun would be hitting the face of the stone but not making it into the carving you might try lightly spritzing the entire face of the stone with water from a spray bottle. You would expect the water from the sun-lit portion of the stone to evaporate faster than that in the carving. There might be a window of time where only the carving is damp and therefore darker than the face.
I love your Phebe Starling photo. What a great name!
