Here is the full list of all 9 Gem Hunter achievements.
Fought and killed Dezravella at the Neon Outpost
Fought and killed Petrichora in the Innerworkings
Found all of the Fox Emissary's Secret Artifacts
Beat the game without dying
Found the Tree Launcher Ammo
Displayed loyalty to the Fox King by killing Ravanna in the Ascendium
Fought and killed the Fox King in his Acropolis
Betrayed the Fox King and aligned to Ravanna's cause
Collected all 28 Diamonds and Beat the Game