Originally Posted by
Vasilis
Urushi might sound "natural", "environmentally safe", "organic" and all that green popular stuff but it's actually way more dangerous than acetone or anything else you can use as a sealant, if it comes in contact with your skin or even vapors from it! Urushiol, it's worse than poison ivy, I've learned that the hard way.
For your choice of wood finish, it says "it cleans easily with warm water", so, even dry, I wouldn't trust it for stone that can't work unless they are wet. Anything water-based would probably not do a good job.
The by far easiest way is to get a transparent acrylic varnish spray; it smells the first couple of minutes, it's dry in half an hour, and a couple of hours later you can leave your stone on any surface of the house without worrying that it will get glued on it. Nail polish works as well but it's less cost effective. If the stone was some thousand dollar stamped one of a kind piece, you might want to consider urushi but for anything else, all you need is a ventilated room or even better, the outside to apply, and in less than 5' the smell will be almost gone, and the varnish stays for years, a can of acrylic spray lasts for many stones and many hands.
Shellac is a good idea as well, but again, it takes some time to prepare if you do it yourself and apply. You will probably pay more for it too.