Oops my bad, I got the emeralds and rubies switched up. Diamonds are significantly rarer due to their weight in the RandomSpawner, they also only spawn 50% of the time. It's also worth noting that the GemSpawner only falls in the highest tier, so a monster must have > 2000 starting health, even then if it's pushed to the Bundle Array it's not a guaranteed spawn, and even still it's not guaranteed because of its weight and spawn chance.
I'll have to take a look at how commonly it actually spawns, but from my maths it seems pretty rare.
EDIT: I just tried it out: I bound a key to spawn the GemDropper and it took 248 tries to spawn a diamond, I'd say that's rather rare for something that only drops from the toughest enemies.