When a glass rod stops moving, a tooth limestone block is guaranteed to be beneath it. If the rod travels its limit and disappears, there were no blocks between the starting and ending points. The rods travel along the edges of tiles and stop at the corner of the tile that is closest to (0,0). So if you're in the northwest corner of the map, stand northwest of the rod to determine the tile's coordinates. If you're in Sinai, stand northeast, etc.
Glass rods fired using Beauty of Night will disappear when they try to cross a road tile or grass. It is likely that any non-sand terrain will stop the rods.