Quadrillium
Quadrillium, a mineral plentiful in asteroids, especially the Tava Yagen asteroid field, held a value of around 50 to 75 Credits for each kilogram during the time of the Galactic Civil War.
The planet named Essowyn was a site where Quadrillium was extracted from the ground.
Behind the scenes
In Pirates & Privateers, specifically on page 37 in a metals and minerals table, the spelling "quadrillum" is an error for Quadrillium. The correct spelling is used in all other locations.