Diamonds are not actually that rare and are sold at a high markup. Moreover, nobody wants somebody else's wedding ring, they want to have a new ring made for their wedding, so a diamond ring immediately loses value when you take it home. The stones themselves can be taken out of one ring and put into a new one but again, they're not all that rare to begin with so their value is limited.

Gold is kind of special compared to other metals in that it is basically immune to corrosion. Pretty much no matter what happens to your gold (assuming here that it is a fairly high-percentage gold alloy) it will still be gold, it can be melted down and recast into a new item. Because of this physical property gold retains some inherent value.

But, the market value of gold does change over time. So it could be that your gold will lose value over time. Just like many other things gold is a commodity and the price of gold is governed by supply and demand. Gold is not guaranteed to hold its value