The large purple crystal can be found in the cave to the north of Quarrystone Basin. When near the crystal you will receive the message "This appears to be a good spot to use the Darcomecron." Use the Darcomecron for completion credit. Now head south of Quarrystone Basin, there you will find the ruined camp of Felix. Near Felix's body you will receive the same message. Next continue south and follow the road to the east as it climbs upwards. At the top of the ridge you will find the Old Idol and your final objective. Return to Bartholomew Cleary at Quarrystone Basin to complete the quest.