Post-transuranic alloys, or PTUs, are ultra-high-strength alloys made using stable isotopes of very large atoms. Creating post-transuranic elements requires vast amounts of energy, and building and repairing structures made of the finished alloys is equally hard. PTUs are therefore extremely expensive.
Annie-plants make heavy use of PTUs, presenting something of a bootstrap problem since they are the best source of the power required to make more PTUs. Large annie-plants are by far the most expensive component of a given structure; annie-plant damage can total a warship.
The ancient Oafa civilization was able to make PTUs more efficiently than current galactic civilization can (or they had a staggeringly vast industrial base), and used them as bulk structural materials. This makes newly discovered Oafan relics, especially ship hulls, a fantastically valuable resource.