First of all, thanks so much for playing!
To unlock Fern, there isn't any special trick, as no turtles or accessories currently boost restore cell spawn rates. The best way to improve your odds is to use Caelo and maximize your light radius so you don't miss any restore cells deeper down. The game uses a pseudo-random system to prevent runs from being ruined by bad RNG. At minimum, one restore cell is guaranteed to spawn every 20 cells. With the game going 433 cells deep (excluding the first 20), you're looking at at least 22 restore cells per run even in the worst-case scenario. Typically, by the time you reach the Magma zone, you'll have around 30, and with good visibility, squeezing out 5 more from there is definitely within reach.
On gems, I actually agree that sapphires feel a bit too rare, since they currently only spawn in the Ice zone. I'm considering adding them to the Magma zone as well to help balance things out. As for diamonds, they're meant to be more common since the diamond pickaxe eats through them pretty quickly. If they were any rarer, the pickaxe would become too costly to be practical, so I think their current rate is in a good spot.
And I love your suggestion for the Halo hat! I originally had it hovering slightly up and down, but that didn't feel quite right. A rotating yellow to gold gradient sounds like an awesome visual, way more dynamic than a static bar.
This kind of feedback is exactly what makes working on the game so rewarding, so thank you!