Nice job on this one. I did a similar sand shot for a previous macro challenge & really enjoyed seeing this done better than what I was able to accomplish:
Regarding the focus, sure the marble may be towards the front of the focal plane (very difficult to hit that same spot time after time), but I think the focus is fantastic since the marble and the stationary grains around it are quite sharp and the far part of the ejecta that is not on edge (the most important part imho) is going right through the center of the focal plane.
Stopping the grains with a shorter flash might have been interesting, but I'm not sure I'd like that more than the texture you captured here with the sand's motion blur. I bet you have some very cool out takes from experimenting with the settings.
Kudos to you Bob for having the perseverance to pull this shot off so well! I think capturing sand drops makes water drops seem like an easy point and shoot.
The symmetrical composition is perfect for this subject. This is really a brilliant capture of the action. One of the few, if possibly the only, image with sand in motion. (I didn't count the hour glass images.) My favorite in this challenge, which included some outstanding images. Hope to see this ribbon! (10)