So Angus at Meandering Banter built this doo-hickey that can turn any list into a bit of code for random table widgets. Below are buttons that should each generate a random gem, based upon but not completely mimicking the charts in the first edition Dungeon Masters Guide.
For instances where you have an odd valued gem (150gp, 400gp, etc.) just use the next lower amount assume an unusual size, a nonstandard shape, a unique coloration variant, etc. By the way, I tend to imagine Huge gems as fist-sized, Very large gems like golf-balls, Large gems like dice, and default gems as roughly dime-sized. But your campaign may be more realistic than mine.
If you want the actual code to put into your Digital DM Screen, here's a file with all of them.
Much of the time in my D&D games I treat gems as simply a nice portable way to score GP and XP, but sometimes you want to know more specific details. For example,
- Spell components: black pearls for animate dead, diamond for bringing people back to actual life.
- Tricking people: A fool might buy a flawed gem for more than its worth, or think a high quality rock crystal is actually a diamond.
- Known prejudices for specific stones: IIRC Lord Dunsany says gnolls love emeralds and and Gygax says in the DMG that dwarves don't care for pearls. Important to know when negotiating prices for ransoms with kidnappers.
- Personal preference: I knew a player once who liked to name her PCs after gems and then call dibs on all examples of that type.
And it might just be fun for a player to find out the details of those three gems worth 100gp sitting on their charsheet.