To get it, Twinsen follow the road to the left of Dark Monk's chapel. The road is tricky, and Twinsen needs to jump to rocks moving in rivers of lava, but at the end of it he finds the crystal. To get a piece of it, he must use the pick-ax.
"This piece of the Crystal of Celebration is of unmatched purity. It will allow you to operate a pisto-Laser."