Doesn't a diamond ring lose value once it is sold, though, in terms of re-sell value? A ring worth one bitcoin is not going to lose value once it is sold (aside from market volatility).
(A paper wallet is a paper wallet even if it is written on gold in tengwar runes that only appear when subjected to fire)
However, a ring inscribed with a private key controlling one bitcoin will lose its value immediately, because whoever comes into possession of (or ever sees) the ring will move the bitcoin.