I agree with MiceOrMouses that the concept works (although I do not like mindless hoarding when it takes a long time, so if the game was expanded, it would probably need some other mechanics and surprises) and that the music is great.

I have some more specific notes, generally minor, except maybe the two about the speed characteristic and ending.

Remarks

1. The game takes QWERTY-only input, which is a problem for AZERTY layouts and others (this is the fourth game in almost a row that this happens to me! XD). Going upwards is thus for me the equivalent of Z, which is somewhat nightmarish (although I managed to survive this trial). X) I do not blame you, it is easy to bypass this aspect without thinking about it.
   You can program so that the physical locations of scancodes are taken into account (instead of the symbolic characters), or at least activate the arrow keys as alternative commands.
2. The consideration for the constraint is a bit far-fetched in my opinion. I mean, ‘dying’ when your health reaches zero is the standard behaviour of the notion, and not a twist on it. For it being a tool would mean that it is useful in some other fashion than mere survival. (Now, you could argue that the drill is a literal tool, but hey, let’s not split hairs. ;))
3. Very small detail: if you click on a button during a fade, you get the sound, but no effect.
4. Characteristic balance: it feels like speed is almost useless, as you spend most of your time drilling from tile to tile. It would make more sense were there already empty galleries underneath, so you could take ‘shortcuts’ to other drilling spots. It is true that you can retrace your ‘steps’ in your own tunnels to find another spot from where to start drilling again, but this seemed like a bad strategy to me in practice.
5. A thought that comes to my mind: is the choice of green, red, and blue for the more valuable minerals some kind of allusion to The Legend of Zelda’s rupees? ;) Although the increasing order seems to be red-green-blue here, instead of green-red-blue.
6. I like how the simple animation of the drill renders well, by switching two nuances of grey (no pun intended, I have no sympathy for smut). :)
7. There does not seem to be an end to the game; it could have been a good idea for something to happen when you have upgraded everything to the maximum or are able to dig deep enough. (I have tested both — discovering that maximal depth is 119, exploding right after by lack of fuel —, and nothing seemed to be triggered. I was hoping to encounter some mining dwarves from Dwarf Fortress or the like, but alas, to no avail.)

Anyway, I liked it, it is simple but efficient.

This game is pretty good! I can see myself spending a lot of time into it. I feel kinda bad that this hasn't been seen by a bunch of people, but I hope it does sooner or later. The art is simplistic but sometimes just plain a simple is better than complex, and the music is also pretty good.

Overall, a nice game :D