RAW: DMG 133 says gemstones weight like coins, 50 to the pound.
Note that in real life 50 to the pound is only about 9 grams. That is a very small mass!
Real life tends to handle things differently for gold coins:
The 1 oz American Gold Eagle (diameter 32.7 mm) is composed of one troy ounce (approximately 31.1 grams) of gold. The silver and copper add approximately 2.8 grams, bringing the total weight of the 1 oz American Gold Eagle Coin to approximately 33.9 grams.
Which is more like a bit above 11 gold coins per pound, and would be exactly 16 to the pound if they contained only the gold. So, D&D gold coins are quite smaller than "real life" gold coins. Think of them more like being 22 mm in diameter, much closer to the 1/4 troy oz american Gold coin. Basically, D&D Gold coins are not at all looking like the typical "dragon horde, massive gold coins shown in fantasy books and movies, but more, like, pocket change sized.
So, that 50/lb can pass ok for gp (D&D 1E had 10 gp to a pound, btw). But you just know that it can become completely ridiculous to just assume that every small gem and jewelry will also weight the same as those smallish coins. It might be better to treat such treasure as art objects instead.
For example, searching the web, for those very typical real life blue quartz cristal pieces, commonly sold at reasonable prices (30$), weighting from 1/4 to 1 pound. Not 1/50 of a pound! However, a real life typical 1 carat diamond weights only 0.2 grams:
meaning there are over 2000 diamonds to a pound, quite negligible!
A ring can weight next to nothing too, or it can be relatively massive depending on shape, but on average they using 50 to a pound is very reasonable. Earrings vary in weight more, but keeping with the RAW 50 to a pound of coins will serve well also there. The idea of 5e RAW is to KISS it a bit. Keep it simple and moving fast.
Necklaces can vary even more, especially when made from cheap materials. A gold necklace is usually quite thin, lightweight. A primitive stone necklace will weight a lot more and be much thicker, but still it won't weight 5 pounds: jewelry is meant to be worn, and who would want to carry some kind of musculation weight around one's neck? So jewelry is always quite light. Maybe 1 or 2 pounds for a really big necklace like say those thick and long hip hop punk rapper "gangsta" necklaces.
So, RAW won't help you for weight of art items and non-tiny jewelry. AFAIK D&D 5e has no such table, and it would be very pointless anyway, because there is way too much variation between different versions of the same type of item.
So, just Google search as if you wanted to buy some jewelry and find one that is "meh, close enough!", check its weight, round it to something simple, and there you go!
In any case, maybe your players love to wall up their body with all kinds of bling, but the fact that they surely hate being encumbered is not a "maybe". So, I recommend to just let them, and to err to the side of "jewelry items are probably somewhat bit lighter than in real life". It's not as if having "moar bling" on them gave them any more bonuses.
Don't bog down the game with some rings weighting 1/50 pounds, some other "thin" rings 1/100 pound, and some other "big" rings weighting at 1/10 pound. That kind of micromanagement is just plain bad. Anyway if they wear a lot of rings, it would tend to average out anyway, right? So, one weight for tiny items: they count as the weight of 1 coin, as per RAW, end of story.
This is a house rule but it is in the spirit of D&D 5e "simple rules": for bigger art or jewelry items, find a single and simple "base weight for the typical thing", then AT MOST define say these 5 categories of weight:
Small and Thin: 1/4 the weight
Small or Thin: 1/2 the weight
Normal: The normal base weight for that type of item
Big or Thick: x2
Big and Thick: x4
Note also that solid gold is about twice the silver weight.
Personally I stopped bothering after complaints from my players.
Nowadays instead of a long list of every valuable item in the loot, I just write something like this:
Loot: 50 art (finely painted figures, coral jewelry, etc.) [Each: 10€, 1/4 lb]
It saves TONS of DM prep time and game table time. If a player asks me "Is there a coral comb in there?", then I roll with better than normal odds or just flat say yes.
Goal is to have fast paced fun, not accounting headaches.