Fantasizing about laser engraving on tungsten or diamond is a lot of fun but I've been thinking about really doing this. Clearly a major issue is that you need to DIY not pay someone else to do it because if you pay someone, you cannot ever quite trust your key. So it seems to me that the most practical solution is to buy 2 "blank" casascius coins and a hand engraver (about 3 BTC). Engrave the private key on one, the public on the other and then glue (or maybe solder) them together so the private key is inside. That would survive most fires... would filling the engraving with solder increase the longevity by stopping rust?
Now we are talking!
Well, Casascius coins are made of brass, which corrodes relatively easily, and melts around 900°C if I remember right.
Just water won't let brass rust, I think. The quickest way would be to let brass touch some other metal which is
either a lot more
or a lot less noble than brass (copper and zinc). Have both metals touch water, aka electrolyte, and voila, have a free battery element. The lesser noble metal will dissolve.
Get a small disc (aka coin blank) of stainless steel, engrave the priv key into it, solder it in between two coins. Or in between one coin and a copper backside with a recession.
Bonus points for engraving the firstbits onto the outer side.
De-soldering is easy to get the key, but will be destructive and noticed. Not that it would help a lot at that point, if the cold-coin-wallet is stolen..
You will be able to engrave any metal with a cheap tungsten carbide tip for a dremel. Or just bring everything to a small jeweler's shop or the like. If there isn't a young guy playing on his smartphone behind the counter when you enter, you should be fine. Relatively! :-)
Ente