Safety and security of the digital hardcoin

The first rule is that the coin is unpowered, and inert. Second, the coin can have a unique stamp. Third, the coin has some good metal in it, making forgery expensive.

When unpowered it is still real money, though buyer should have open access to read value by powering the coin and requesting public information from it.

When then is the disutility between the digital hard coin and a gold coin? Digital coin needs low cost value readers. The digital will wear out eventually.

Different exchanges could produce their own versions.  Small value coins, say up to a hundred, no password required, good to go.  And they talk to the register, can take change and be returned.  High value coins storing thousands might have a simple four button password arrangement, making them slightly bulky.  Manufacturers guarantee them to be valid Bitcoin accounts and sell them, adding mechanical durability, embedding the chip, and adding precious metals.

The chip issues a serial signal, its program is read only, certified at the manufacturer. It is a two port coin, power and signal; the case providing neutral.  It can be laid simply on the reader. At the gas pump it works like a credit card, but can be anonymously traded cash. All the registers will know how to talk to it.  Industry can try out mechanically raised lettering for serial proofing of their quality.

Consider the high end digital coin, encased in gold.  It can tell you what its nominal weight should be and who manufactured the thing.  The extra intelligence is less than a buck, and rides along with the gold. The brains make no exchanges, gold speaks for itself, but it makes the gold much more liquid since its contents can be truly reported anywhere there is a low cost reader.

Common everyday hard coins.  Really just instant ATM cash accounts one might by, just to use the payment system.  At the low end, they are disposable, simple plastic disks with the bare minimum metal for connections.  Use them up then return them via the merchant chin for recylcling.  They can be dated at the low end, spend and return within some time period.

Retail advantages.
Mainly the low cost reader and no need for connections or verification.  The coin secures itself, and the readers should cost less than a cheap calculator.  Unlike a credit card, I can grab a small valued digicoin and loan it out for errands, just like cash.
Why not?.