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).
Yes, a diamond ring cannot be resold, since the price of diamonds has been decoupled from the difficulty of getting them. You're not paying that jeweler for the diamond on the ring; you're paying him a special fee for being a De Beers recognized diamond merchant. A bitcoin doesn't have that problem.
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.
