One of the essences in Roper Klacks' laboratory that, when mixed, create his potions. Each one can be smelt, handled or listened to, in order to match it to the pictures in Klacks' spell book.
This isn't technically an essence - it is a catalyst used in making all the potions. It smells slightly earthy, feels cool to the touch, and 'sounds' like faint bells.
Obtained underneath the skull in the laboratory.