Colossal Cave
24 Achievements
GOG
Treasure Seeker
Player discovers 5 treasures
24.4%
How to unlock the Treasure Seeker achievement in Colossal Cave - Definitive Guide
This will unlock automatically once you have collected five pieces of treasure. You don't need to have all at once in your inventory. I had died and gone back to the shack, where I left my previous three pieces of treasure. Also, try to go back to the shack often, as there's a pirate roaming the lower levels of the cave that likes to mug you.