Gem Ore - FrogMan650/EndGameEnhanced GitHub Wiki
Gem Ores spawn 2 blobs of 4 ores per chunk between 0 and -64. When mined with an iron tool or better they roll a special gem table 5 times. The rare gems will be rolled for first in descending order and if they all fail you're guaranteed to get a Sapphire.
The rates listed here are per roll, of which you get five rolls per ore.