YO WHERES A DIAMOND?! has anyone actually played the game here and not just jerked it to a full save?
High level Gathering Spots. Usually lcoated near the bosses.
I'm not sure if it's restricted to certain areas, but just warped to the Pyramid Monument and found one first try.
edit: In case you haven't already, buy the Art of Gathering 1&2 from the library for more rewards per Gathering spot.
Can't really tell you more than that. By the tie I finished the game, I had 13 of them in my inventory. Which does appear to be roughly 1/3 as many as I have of the other gems so I guess Diamonds have a slightly lower chance of spawning. But that's about it. Just pick up Gathering spots when you see them and you should be fine.
edit 2: Did some testing.
Diamonds only spawn at Gathering Spots in the Pyramid from what I've found.
Vulcano only generates Rubys, Seacave only Aquamarine and Mountain only spawns Emeralds.
Pyramid can generate all gems, in addition to Diamonds. Seems like the droprate also doesn't depend on the floor or "difficulty" of a gathering spot. I wanna say there appears to bea ~1/5 chance for a gem to be gathered from a gathering spot, which in turn has a 1/4 chance of giving you the gem you want (as long as you are in the Pyramid).
One easy way to force a Diamond is to save infront of a gathering spot and keep reloading until it gives you what you want (Gathering spots are not predetermined and this isn't a game with a consistent RNG seed, so the gathering results will change between reloads)