The Fire Opal is a powerful stone used by the Soul Eaters to attract and control Demons. The original stone was shattered into three pieces by Torak's Fa, in an attempt to destroy the Soul Eaters' power. But the pieces survived, just as powerful. They were wielded by the remaining Soul Eaters in the course of the series.
The Fire Opal, and it shards, are described as very beautiful and fiery red, with a glow likened to fire. It was however cold to the touch, such that Wolf described it as the "Bright Beast That Bites Cold", contrast to the "Bright Beast That Bites Hot" (fire). It was in fact so beautiful that Torak's Fa couldn't bring himself to destroy one piece (featured in Outcast), so he instead concealed it inside the hilt of his Blue Slate knife, which he passed on to his son at his death.
Method of DestructionEdit
To destroy the power of a piece of the Fire Opal, it must be buried in the earth together with a living soul, which must die with it.
Significance in StoryEdit
WARNING: SPOILERS FOR WHOLE SERIES
In Soul Eater, four soul eaters, Nef, Thiazzi, Eostra and Seshru, used the second shard in a demon summoning rite that involved the murder of hunters. This shard was ultimately destroyed when Nef sacrificed herself by pluging into a crevasse with the opal in her hands.
In Outcast, Torak discovers the third Fire Opal shard hidden inside his Fa's knife hilt. This piece is smashed when Seshru the Viper Mage demanded that Torak give it to her. And when Seshru tries to gather the fragments she is shot by Bale in the heart and dies, this piece thus dies with her.
In Oath Breaker Thiazzi finds the last shard of fire opal at a shrine in the Seal Islands. In the final showdown between Torak and Thiazzi Eostra's eagle owl snatches the fire opal from him thus it is probably given to Eostra.
In Ghost Hunter, the Eostra used the last Fire Opal shard to summon the spirits of the deceased soul eaters in order to bring them back to do her bidding. Torak snatched the shard from her and destroyed it, thereby destroying the last fragment of the Fire Opal and putting an end to the Soul Eaters' power.