The Green Crystal is an item that belongs to Number Eight, found in his Loric Chest.
After Eight's sporadic teleportation from India, he, Marina and Ella find themselves in the Gulf of Aden near Somalia. Marina discovers that the Loralite rock is at the bottom of the sea. Eight activates the Green crystal which glows and produces a strong force of wind that creates a crater in the water and the group are able to pass through the water to the bottom, effectively in a bubble of air. At the bottom they are able to Teleport again.
The Green Crystal likely disintegrated with the death of Number Eight.
Following stones such as the Healing Stone and the Xitharis, this item may also have the same attributes as a Legacy.