Crystal Heart is easily one of the most fun abilities to use in the game. It will allow you to fly from one area to another very quickly. This ability can be found in the Crystal Peak which is located to the right of the Forgotten Cross Roads.
Start from Dirtmouth and jump into the well. From there head to the right
Follow the path until you get into a big open room with multiple paths. You are looking for the opening at the bottom right that looks like it is part of a mine.
There is only one path which will lead you to an elevator and then to a blocked path that will require the desolate dive ability.
Note that there are two ways to get into the city of tears
- Requires desolate dive
- Requires the lumafly lantern which you buy from Sly (after you find him) in Dirtmouth for 1,800 geo.
Once you are in there is only one path you can take that leads down and to the right. You will have the follow and platform over the conveyor belts carrying crystals.
At the end of the path you can go into the first door you find on the bottom right and it will lead you into a dark room with a bench. There are only two benches in all of the Crystal Peak and the second one will require a boss fight. I skipped the second bench on my first playthrough as the boss was optional.
Go back the way you came and from there you will climb quite a bit filled with lots of platforming.
Keep on climbing until you make it to an area where you can go up or to the right. By going through the room on the left you will find Cornifer who will give you a map for some geo.
After making your way to Cornifer you can climb two more rooms. There is more platforming and lasers in this area. Just go slow and you will be fine.
From this point go one room to the right and drop down at the first available opening.
Keep following the path down and to the right until you reach a switch. The switch will make it so that you can get back to this area again much faster. You may need it as the next section of platforming is pretty difficult.
At the end of the path you will see a pulsating light coming from the downed mining machine. Go investigate and you will be rewarded with the Crystal Heart ability.
The bridge will collapse behind you and the Crystal Heart ability is how you can get out. Go save at the bench and make your way back to Cornifer. You can Crystal Heart your way back to Dirtmouth from there.