This is what we use to fill chips and holes in slate. This product gets very hard. Probably harder than the slate itself. It's meant to be a permenant fill. The best quality is that it does not shrink while drying. You fill once, let it dry, then lightly sand with a sanding block.
This is not a good candiate for filling seams. Use the bees wax with resin instead.
50 grams per package which can fill a couple of medium sized holes.