How connect unmaterial digital signatures and material gold, I think that it is mistake that crypto can protect gold or diamonds. Just wait and we will see that this will create more fraud.
The gold asset (jewelry, bars, coins, etc.) would have a QR code (asset ID for example) on it containing its description, just as it carries the mark of manufacturer (in case of jewelry), you'd be able to track the history of this asset and verify its meta data and real owner.