The 1st Edition of AD&D Dungeon Master's Guide is a wonderful resource for any referee to possess. In the section "Money (Gems & Jewelry)" there are roll tables  for determining the precise values and types of gems and treasures rolled.

These sadly don't exist for my preferred edition: B/X. Specifically, OSE has this to say on Gems and Jewellery.

Gems


The value of each gem in a treasure hoard is determined by the following table:


Gem Values
d20 Gem Value
1–4 10gp
5–9 50gp
10–15 100gp
16–19 500gp
20 1000gp

Jewellery


Each piece of jewellery indicated by a treasure type is worth 3d6 × 100gp (or more, if the referee wishes, for characters above 3rd level).


Damaged Jewellery


Rough treatment of jewellery (e.g. crush-ing, intense heat or fire, lightning) can damage it, reducing its value by 50%.


~ Old-School Essentials Core Rules under Gems and Jewellery

We're going to have a much easier time lining up the rules for gems than the rules for jewellery. Gems in the DMG are listed in the categories:

  • Ornamental Stones, Base Value 10gp
  • Semi-Precious Stones, Base Value 50gp
  • Fancy Stones, Base Value 100 to 500gp
  • Gem Stones, 1,000 or more gp

Jewellery is resolved very differently however. It is given a d10 value ranging from 100gp to 12,000gp (it's actually a d100 table, but each result is an even multiple of a 10% chance).

This might be the 'or more, if the referee wishes, for characters above 3rd level' that the book talks about. As Jewellery in Basic edition ranges from 300gp to 1,800gp in value, significantly less than the range of AD&D1e.

Interestingly, this lines up with the definition given in AD&D1e for the value of Wrought Gold jewellery, which is found on a roll of a 3 or 4. It might simply be that all jewellery in Basic edition is expected to simply be shaped gold.

This is an elegant solution, and works nicely with the silver-standard revised system I use, where I can simply say that 'jewellery is made of silver'. Though I do like the posibility for lower and higher grade of treausre to exist, and to account for the expert-edition level ranges and their never stated expectation for more valuable jewellery.

So, for gems, simply consult Page 26 of the AD&D1e DMG for treasure the values and descriptions of different gems. I'm not going to reproduce it in full here as you can go out and buy the book if you want this content and I don't want to get into an IP trouble. But here is a list of gems for each category:

  • 10gp: Azurite, Banded Agate, Blue Quartz, Eye Agate, Hematite, Lapis Lazuli, Malachite, Moss Agate, Obsidian, Rhodochrosite, Tiger's Eye, Turquoise
  • 50gp: Bloodstone, Carnelian/Sard, Chalcedony, Chrysoprase, Citrine, Jasper, Moonstone, Onyx, Rock Crystal, Sardonyx, Smoky Quartz, Star Rose Quarts, Zircon
  • 100gp: Amber, Alexandrite, Amethyst, Chrysoberyl, Coral, Garnet (red or brown), Jade, Jet, Pearl (white, yellow, or pink), Spinel (red, brown, or green), Tourmaline
  • 500gp: Aquamarine, Garnet (violet), Pearl (pure black), Peridot, Spinel (deep blue), Topaz
  • 1000gp: Black Opal, Emerals, Fire Opal, Opal, Oriental Amethyst, Oriental Topaz, Sapphire, Star Ruby, Star Sapphire

It also has some gems valued at 5000gp, which can't occur on the OSE gems table. Perhaps I'd have one of these show up if after rolling a natural 20 for the most expensive gems, I rolled again and got another natural 20 (1-in-400 chance).

  • 5000gp: Black Saphire, Diamond, Jacinth, Oriental Emerald, Ruby

Now for Jewellery, falling between 300 and 1800 does give us the nice catch all of "it's all just gold" (or silver, in my case). But I'd like to see some variety given the boundaries of the table are quite wide.

If we look at the original table, 50% of jewellery types that do not exceed 1800gp are gold. Let's keep this number, and divide evenly by the other two, giving us a quite nice table to start with, ivory or silver on a 1, electrum on a 2, and gold on a 3 or 4. However, we want there to be the possibility to get higher grade metals. Jade, coral, platinum, and gemmed silver are all able to be worth 1800 or under (gemmed gold, and gemmed platinum exceed the 1800 minimum). Of these, jade, coral, and platinum are \(5\text{d}6\times{}100\) gp in value, that will yield something beneath 1800 pretty much exactly 60% of the time, so lets give that a weights of \(\frac{3}{5}\) (compared to the 1 and 2 weights we've given everything else). Gemmed silver is the only other result to consider, which is \(1\text{d}6\times{}1000\) gp, we could naievely give this a value of \(\frac{1}{6}\), but 1800 is almost 2000, so we should probably consider how much of the graph is really likely to be under 1800 if all values could generate on the curve (instead of just 6 discrete points). Fortunately those upper bounds are an easy to calculate straight line, so we know that 1800 should be around \(\frac{3}{10}\).

For the same reason we exclude our high end somewhat, we must exclude our low end somwhat also. We will not generate gems with a value of less than 300, and so electrum, silver, and ivory are all slightly less likely. There's an 80% chance that \(1\text{d}10\times{}100\) is equal to or larger than 300 for ivory and silver. There's a \(\frac{35}{36}\) chance that \(2\text{d}6\times{}100\) is equal to or larger than 300 for electrum.

All of this together, gives us the following table, with conditions for a reroll

1d100 Material Conditions
01-09 Ivory less than 1000gp value
10-18 Silver less than 1000gp value
19-39 Electrum less than 1200gp value
40-82 Gold
83-86 Jade more than 500gp value
87-90 Coral more than 500gp value
91-94 Platinum more than 500gp value
95-00 Gemmed Silver more than 1000gp value

Then, as for which type of jewellery it is, we'll just randomly select from the examples given in the 1eDMG.

1d8 Type
1 Bracelet
2 Brooch
3 Crown
4 Earrings
5 Necklace
6 Pendant
7 Ring
8 Tiara

And as a bonus to myself, here's one more reasonable for my adjusted metal values where silver is significantly more more valuable and gold even more so (100:100:100 instead of 10:10:10).

1d100 Material Conditions
01-09 Ivory less than 1000gp value
10-81 Silver
82-87 Jade more than 500gp value
87-93 Coral more than 500gp value
94-100 Gold more than 1000gp value