There are a total of 5 different crests scattered throughout the world. Each one is hidden in a different location, waiting for you to discover them. When you have all 5 crests, take the five crests to Rubiss' Shrine to receive Rubiss' Charm. This is the only way you can acquire the Rubiss Charm.
Using the echoing flute will help you locate the crests. So be sure to have it handy at all times. But if you would just rather have the locations revealed, read below.
Sail west of Zahan to the fire shrine. Search in the upper right corner.
Once you have all the keys, go to Hamlin and defeat the demons.
Tower South of Alefgard. Follow the old man in it to the treasure chest, he is actually a demon you must defeat.
Defeat the Saber Tiger in Osterfair and the King will give you the crest.
Search the bottom floor of the cave to Rhone.