This was quite enjoyable to play. Although I had quite a bit of trouble controlling the jetpack (?), all my hours in Kerbal Space Program seems to have paid off here.
I got to the level with the two buttons that open a door, and couldn't get any further; when landing on the end bit with all the diamonds collected, nothing happened. So it's the last level maybe?
I would have liked to see more level design that forces me to use the reversed gravity -- I spent 95% of the time in regular gravity (just because it's easier to fly), flipping only at the last second to mine diamonds. This might be difficult to achieve, actually, since with a jetpack, you can already move up and down, but maybe a long stretch with spikes across the entire floor could have at least encouraged it?
I will mention that the music got really repetitive really quickly. A much longer loop would have done wonders to avoid that.