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This table is a random Gem Generator inspired by (not transferred from) tables 13B through 13J of the 4th Edition HackMaster GameMaster's Guide (say that 5 times fast!)
That game had a mechanic for altering the value of gemstones based on quality and size. Of course rolling on five separate tables or sub-tables can be a bit much, especially since you'll need to use three different sets of dice.
Nope, one d30 is all you need with a minimum of two rolls and a maximum of four. These tables should pretty much be drop-in compatible for any OSR game. My next few tables are likely to be for other types of treasure.