Crystal egg
From Discworld MUD Wiki
Crystal eggs can be given to Libby Twickle to make custom crystal jewellery from, or shattered into thaum crystals.
Availability
Crystal eggs can be gotten from the nest at the top of a tall pine tree in the area leading to the grflx caves in Skund Forest.
Warning: a fall from the tree can be fatal (at least 3,000 damage, from the top of the tree).