Here is the full list of all 18 Quell Zen achievements.
Merge 6 of the same gemstone at once
Change the color of the same raindrop 5 times during a level
Fill every tile with a gate
Exit the same ring in all 4 directions during a level
Discover a black hole level
Collect 4 jewels
Discover a zen stone level
Complete a stage in perfect moves
Complete a chapter
Complete every chapter
Complete every chapter in perfect moves
Collect every jewel
Collect every zen stone
Light a symbol 10 times during a level
Toggle the same rose 8 times during a level
Illuminate blue, gold, red and green diamonds at the same time
Pass a beam through 4 rings
Use shift arrows to move another droplet