Hook: Wall Medium (Blue Square)The Rakata Mantlepiece (Purple) decoration fits into a blue medium square wall hook.
Hook: Wall Large (Purple Rectangle)The Rakata Mantlepiece (Purple) decoration fits into a large purple rectangle wall hook.
How to Get a Rakata Mantlepiece (Purple) Decoration
Credit Decorations (previously crystal decorations)
The Rakata Mantlepiece (Purple) decoration can be easily bought for 50,000 Credits from a vendor on the fleet. It used to cost crystal commendations, but now can be bought with just credits.
To buy a Rakata Mantlepiece (Purple) decoration, head to the Crafting & Strongholds section of the fleet.
It is bound to legacy, so you can send it to your other characters... but once unlocked, all decorations are available in your stronhold legacy-wide, so the legacy-wide designation is mostly pointless. Unfortunately, due to the legacy restriction, it can not be donated to your guild's bank.