Well, the components of a jewelry price are fivefold (unless I’m forgetting something, which I might) - cost of materials, cost of labor, design, brand, and historical/sentimental significance. You might include transport and taxes too.
Materials: obviously you could melt down the metals in jewelry, extract the gemstones and sell them. And a manufacturer can’t sell an item for less than the materials cost and stay in business very long. Diamonds and gemstones are somewhat individualized, but there are standard grades and colors and sizes of gems (not unlike oranges or other agriculultural commodities).
Labor: these days, a lot of jewelry, even if designed in developed countries, is manufactured in developing Asia - China (inc Hong Kong), Malaysia, Philippines, etc.. Those costs tend to be fairly low, but they do factor in.
Design: in developed countries, this is where value added can come in. It’s hard to command a premium on a plain gold band, since there isn’t a lot of differentiation. However, a good design will attract a higher price. Design is an interesting thing, since it adds value, and is a finite (not everyone can do good design) but non-rival resource (one person designing well doesn’t mean that fewer other people can design well).
Brand: OK, if the plain gold band comes from Tiffany & Co., or Cartier, etc., you get a premium.
Historical/Sentimental significance: If Queen Anne owned it, it’s probably worth more than all the others with the same quality. If your grandma owned it, it’s probably worth more to you than others are willing to pay for it.
So basically, if you want high commodity value as a percentage of your jewelry, you want to go for non-sentimentally-significant, unbranded, plain design jewelry. Jewelry without gemstones is probably better, because you would have to sort and grade them, whereas gold can be melted down and separated.