Yeah, it was a fun challenge. I love exploration in games like this.
Like Knytt Underground, which isn't so collection based, but has a bunch of cool secrets if one goes looking for them.
That said, it honestly leaves me even more curious.
I counted the diamonds in that map several times, and there's only 55.
Which implies that the last 4 don't even exist, possibly the keys were added up with the tally?
Also that either the map is missing one, though I can't tell where it would've been from the map... or one of the ones I picked up got counted twice?
I made a script that counts all the diamonds in the level (so I wouldn't miss any when counting by hand) and suprise, suprise - it also counts keys :) Should've guessed that if class extends from Diamond (Key extends Diamond class) that key will also be counted as diamond instance. However collected Key won't count as a Diamond because each item has it's own collection method that does not depends on item type.
Hats of to you, and mad respect for your curiosity. Sorry for my mistake!
The 56 is maximum you can find, one diamond is duplicated. Therefore you have made 100% progress.
