The first time, I devised some method of multiplying d20's and d10's that generated rich enough gems to insta-bump the party a level or so.
The second time, I thought "Let's roll 3d6, take the lowest as the number of zeros, and d10 as the lead digit." Then after rolling a couple of gems, "OK, lowest minus one."
(Later, I figured out that the first method gives an average gem value about 50,000 and the second, 5,000. Lowest of 4d4 x d10, however, gives an average value of about 700. All heavily skewed, ofcourse; the typical gem will be closer to 50.)
And then I really wanted a gem table, and of course because AD&D or 3e is not good enough I had to roll my own. Including fantasy gems. It's weird that all the gem tables in D&D have not included otherworldly gems. Like the glowing green "gromstones" I imagined as a teenager, or some possibilities that arise from the infrared spectrum. And there are real stones that sound like the products of fantasy - iolite (renamed here "Jolite" to stop being misread as "LOLite") and alexandrite.
The true gems are really rare (only about 6% chance) but you can bump things up for richer hoards by making some or all the dice d6, maximum 4.
Uncut gems are a cool find. Will they discover a flaw, or a rare inclusion? Can scrying magic bump your sales price?