Here is the full list of all 18 Quell Zen achievements.
Light a symbol 10 times during a level
Toggle the same rose 8 times during a level
Collect every zen stone
Complete every chapter in perfect moves
Collect every jewel
Complete every chapter
Pass a beam through 4 rings
Illuminate blue, gold, red and green diamonds at the same time
Use shift arrows to move another droplet
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
Discover a zen stone level
Collect 4 jewels
Complete a stage in perfect moves
Complete a chapter