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I used epoxy the first time and it did not work, then I switched to gorilla glue with superb results.
The glue expands and fills the uneven areas of the stone.

I have a can of cashew lacquer and that is what I use for sealing my stones. I have used shellac before on many occasions and it works fine.

You can lacquer the stone, and if the crack goes trough the whole stone mount it on a base.
Usually stones will not split on the cracks unless subject to adverse weather conditions such as extreme...