Of all the possible treasures he could have found inside the vault disguised as a water tank, he found the most precious of them all. The dagger was a symbol of their friendship, but contrary to a token piece that could only be used as a memento, it was a useful weapon that would certainly make his life easier and more than likely, save his life.
The colors, the design and even the type of weapon were a clear sign that their friends knew him very well. As an expert archaeologist, Uriel needed but a quick look to know that although the dagger was crafted with Medusa, Luna and Nika helped in its creation. All of their feelings were poured on the weapon which made it priceless to him.
That appraisal was also backed up by the fact that he was able to store the dagger inside his museum, but it wasn't until he read the carvings on the pedestal beneath the dagger that he truly grasped how priceless the dagger actually was.