Then the thing can be sealed in such a way that if anyone opens it, you destroy the physical bitcoin. You'd have to re-create the coin, but then it would be obvious.
This part is troublesome. Crooks will always find out various methods, to open the coin without physically destroying the Bitcoin. Also what if the manufacturer of these coins copy the private key to somewhere else, before entering them to the coins?
Yes, so, if the coin were purely physical, like casascius coins, people have already managed to expose the private key without destroying the hologram sticker.
But if the physical coins were actually chips sealed in epoxy or something, there is no way to x-ray that, and the only point of contact would be the terminals, but all the chip can do is sign a message.
Sure you can probably empty or sweep the bitcoins away, but anyone who can see the blockchain will know.
I think, that's the main issue, the only sure fire way to know if any physical coin has real bitcoins is to check the blockchain, therefore internet service is required. There is only a token amount of certainty if you have an offline blockchain that is not updated in real time.
But if a particular bitcoin address has held a value for a long time, it might make sense to assume that it will continue to have that balance.
So there are two ways to make physical coins:
1. low tech way, just a coin secured by how it is sealed, brittle, easily destroyed, not easy to recreate, and has low value so it won't be a target since it costs more to "open it up".
2. smart chip or something and only a serial terminal to communicate with it. Think of yubikeys, for example.
You combine both methods to produce a smart chip that dies easily if someone attempts to open it, rendering the coin useless.
For my own personal use, I could get a micro SD card and seal it in it's original container, then sink that in a small cup of resin. When the thing dries, I'd have to carefully break it out. Those memory cards can last maybe 10 years, more if not use and sealed like that.
Of course, no one else would believe that this little blob can possibly contain the private key.
I'd still pick bitcoin over gold, at least for the next 5 years.
