It measures 30cm-wide and 67cm-long: dwarfing the current world's biggest Pearl of Allah which weighed 6.4kg and is worth US$35m.
Tourism officer Aileen Cynthia Amurao said: "The fisherman threw the anchor down and it got stuck on a rock during a storm.
"He noticed that it was lodged on a shell and swam down to pull up the anchor, and also brought the shell with him. This was a decade ago and he kept it at home.
"He didn't know how much it was worth and kept it tucked away at home as a simple good luck charm.
"We're waiting for authentication from Gemologist Institute and other international authorities."