Complete the Objectives
This is the ONLY way to earn more free crystals in Fish Out of Water. The added objectives make the long-play of Halfbrick's latest iOS release more fun too. Why wouldn't you want to earn fish-gems the honest way? It's more fun to challenge yourself to meet specific goals or progress deeper into the gameplay mechanics. That said, there are points to understand about the crystals and how they work.
Remember, when you complete achievements, you'll level up and that gives you the choice of three different chests. Within you'll find the crystals you want, but you'll never get there if you remember to reference the objectives. Tap on the flag in the bottom right corner to pop open a banner describing your current achievement objective.
Once you've earned some crystals, you can combine pieces for different boosts. Maybe one combination of crystals will give you a perfect 10 from a difficult judge, or and add 100 meters after you jump. These boosts are valuable and using them in can net you the top of the leaderboard. Mixing and matching colors will yield varied results, some more powerful than solid-colored fishes.
Remember extra crystals can be bought in packs of 50 and 250 in-app, but earning your valuable jewels will feel a lot more rewarding, especially if you defeat the field of fish-flingers for that day on the leaderboard.