Sky Gems are blue triangle Gems which are found in battle castles.
When selected, they will allow you to hover across the world depending on the level of which you originally took flight from.
Using the Sky Gem
Right-clicking with the Sky Gem selected will increase the height of your flight, whereas left-clicking will lower you.
Selecting the Gem whilst falling will hold you in midair, preventing fall damage.
There is a trick which can cause you to fly at high speeds; by quickly alternating between left and right-click, you can propel yourself faster during flight.
When descending, alternating between selecting and unselecting the Sky Gem to lower yourself will save durability on the Sky Gem.