These are part of a main story quest given by The Inquisitor Mendoza in the beginning of part two in Risen.

There are a total of four scattered around the Island.

You also get a broken Crystal disc from the Inquisitor after you have become a Warrior of the Order. To repair it you have to go to Eldrics hut in the north east of the island.

Two discs are given for completing quests given at the Bandit Camp.

Another one is given after completing the buried treasure quest, where you will find it in a cave after you have defeated Romanov and his men.

This makes for a total of five discs. They are used to open the door in the volcano which forwards the story.