These four stones were located in the The Tomb of Amyana. They are all fist-sized crystalline rocks, of four different colors, and to Essence sight they appear to be attuned to certain Essence shards.
Largo's Castestone[]
This opaque white crystal responded to Largo's strength, being crushed to pieces by his hands. Inside was an obsidian lion figurine. Upon being freed from its white shell, it raced off out of sight and disappeared out the watery doorway. It then somehow reappeared, inside the white crystal, in Largo's pocket a few minutes later as he was searching for it.
The castestone's final resting place was alongside several other small black lion statuettes that were in Arcadi's tomb. Largo pushed these into a Dawn caste symbol to activate the elevator.
Amber's Castestone[]
This green crystal responded to Amber's voice. With a fair amount of experimentation it became clear that it glowed with certain pitches. Further experimentation showed that certain progressions of notes kept it lit, and eventually a song was played to which it kept its light constant. This song was recognized by Dax as an advertisement jingle for a famous casino-brothel in Coral known as the Nightingale's Roost.
The castestone's final resting place was in a slot on the Portrait of the Champions of the West, where it could then show the complete history of the Solar Circle.
Dax's Castestone[]
This violet crystal seems to glow in darkness and shadow, during which only parts of it are lit. This is more visible at night by the light of the moon and stars. Extensive testing reveled that the small internal lights shifted around with different orientations of the stone, and by rotating it just right the lights aligned into a bright, clear arrow.
The castestone's final resting place was in the dashboard of the Coatl, where it points five arrows in different directions.
Fern's Castestone[]
This red crystal seemed to flicker as Fern attempted to channel Essence into it, but would not produce a steady glow. When she cast the Incantation of Effective Restoration on it, it seemed to respond to the act of shaping Essence, but did nothing more.
Later, when she was confronted with a deep fear (Saffron Dolphin heading for the Realm), the stone began to display a Twilight caste mark. She pressed it to her forehead, where it began to burn and melt into place, eventually forming an opaque red-amber visor or blindfold that grants her permanent Essence sight at the cost of normal vision.
The castestone's final resting place is across Fern's eyes.