Atlantean amulet
The Atlantean Amulet only appeared in the episode Atlantis SquarePantis and is a golden amulet that was broken in half. The first piece was displayed in the museum of Bikini Bottom. The second half SpongeBob and Patrick found in a cave. It was soon put together, and it presented a van to Atlantis.
Looks
It looks like a golden piece that belonged to Atlantis. It has a big A in the middle. It had a great sparkle coming out of it, and it was presumably made of gold.
Disposal
The Amulet was thrown into a dumpster by Lord Royal Highness. He didn't want more "visitors" like SpongeBob, and Patrick.