It is likely that the glass pebbles cannot be returned to their original color, as the soda likely cooled them rapidly, causing contraction and the residue to be drawn into the glass.
You may try soaking the pebbles in a tub of glass cleaner, as this might help to draw out the residue. If the color does return, rinse the pebbles in clean cool water before returning them to the fireplace.
Collin C. - NFI Master Hearth Professional
on June 25, 2013