I found 11 out of the 12 gems, but I haven't been able to locate the last one for some reason.
EDIT: Much more findable with the new changes, thanks! I did expect something to be in that general area, but I didn't think to find there even then.
Very interesting game and had a lot of fun playing it. That being said, I do agree with the previous comment that...
(Spoilers)
...the fact that there's nothing that felt like an Absolute Final Thing is a little bit of a letdown. The Dark Dragon Statue or Unicorn endings are probably the closest ones, but only because you need a lot of effort to achieve them rather than any sort of narrative closure, because the end result is usually more or less the same. (After doing those two endings, I just stuck with actually using the Ending Hax Tool, lol.) Nonetheless, I still had a lot of fun playing around with different fights and seeing how things interacted.
Getting the Unicorn ending without the tool is definitely doable, but you need the right strategy and a lot of luck on your side; it took me about an hour of resets on the Birdman fight to get through because of the crows giving the Blind debuff quite often. After that I just spammed Summon Crow and got through the Imp fight fairly easily.
Just a little something I found out by accident and wanted to show.
Apparently, if you kill the boss immediately after your death... This happens.
(Note: Rapid fire being possible is only due my computer's performance slowing down by like 30% when I try to record on it, the game runs at normal speed otherwise)
(Also, love the game, it's wonderfully done for being a game jam entry, just thought this could be fixed, even if it was hilarious to me for some reason the first time I encountered it)