Jewelry at retail cost is not a very good investment over the long term. The markups by retailers and designers raise your cost basis so as to diminish returns. Even at wholesale prices, it’s not something to count on it put your children thorugh yeshiva and pay for chasunahs.
At best, the long term prices for gold stay about the same, after adjusting for inflation. In that sense, some, depending on their tolerance for volatility, use it as a hedge against catasrophe. Harder to tell for diamonds as the supply might be artificially constrained by DeBeers.
In times of crisis, human greed, materialism, and ego make gold and silver, diamonds an other gems commodities that can be traded for something, even if not equivalent in cash to what you originally paid for the item.
It all depends on the individual investor’s needs for portfolio diversivication, tolerance for risk, utility of marginal return on investment, and other factors. The intrinsic value of gold and diamonds remains fairly high because there are few functional substitutes in industrial applications. But the intrinsic value is not the same as market price. Even if you feel like you need and want precious minerals and gems as investments, you have to “know the price of tomatos” before you buy them, so you can tell if you are getting a reasonable price or not.