How to open Aster Strongboxes in Kingdom of Amalur?

I searched internet for an answer but all I found was that I need to find a gravestone that gives me "half an amulet" which then starts a questline that will reward me a key to these chests.

What is the name of the quest line and where can I find these gravestones?

Thanks!

Name of the quest line is The Aster Inheritance.

Heric Aster's Grave

Heric Aster's Grave is located South, South-West of Helmgard Keep, against the Western base of Skycrown mountain.

Bodan Aster's Grav

Bodan Aster's Grave is located South of Dolve Wayle in the Tala-Rane section of the Plains of Erathell.

You can find full walkthrough for this quest along with more details about each gravestone location here.