Hook: Floor Medium (Blue Square)The Large Crystal Formation (Purple) decoration fits into a blue medium square floor hook.
Hook: Floor Large (Purple Square)The Large Crystal Formation (Purple) decoration fits into a large purple square floor hook.
How to Get a Large Crystal Formation (Purple) Decoration
Flashpoint decorations have a chance from dropping from a certain flashpoint. They are not a guaranteed drop, but have good chances of being looted from the final boss and a high chance from the bonus boss. Hardmode flashpoints have a higher chance of dropping a decoration than storymode flashpoints.
Approximately one decoration is dropped per hardmode flashpoint, and each flashpoint drops a different type of decoration. The Large Crystal Formation (Purple) decoration is dropped from the The Battle of Ilum Flashpoint.