If you\'ve already painted the rest of your stones, it\'s unlikely that you\'ll fill those gaps without damaging the rest of your paint job.
I\'d suggest making a wash and using that to fill those gaps. Dish washing liquid will allow the wash to wet the surfaces and flow into the gaps. Acrylic floor wax will help to draw the rest of the wash into those gaps. If not floor wax, then a bit of PVA glue in the wash will help that effect too.
Here\'s what I\'d try first:
1 part black paint, 4 parts water, 1 part floor wax.
or
2 parts black, 8 parts water, 2 parts PVA glue, 1 part dishwashing liquid.
This is fairly thin, so you might need to use a couple of washes to make sure the black covers properly. And next time, wash the stones before painting them.
