Here is the full list of all 13 Gemstone Keeper achievements.
Defeat first boss.
Kill Five Creatures without getting hurt.
Defeat second boss.
Defeat third boss.
Defeat fourth boss.
Defeat fifth boss.
Kill Fifthteen Creatures without getting hurt.
Kill Thirty Creatures without getting hurt.
Collect 50 Unique Gemstones
Collect 100 Unique Gemstones
Collect Every Unique Gemstone.
Purchase every weapon and ammo type.
Complete the boss rush.