Spirit Stones.

Raven explained some of the things she had found reading the book.

First off, she took out a few crystals the size of a clenched fist. With its lackluster and rough surface, one would think that it was merely a raw crystal, but the truth was far from it.

The burning white light emitting from its core and its warmth in one's hand was proof of it.

"From the illustrations in the book, this is supposed to be Medium grade Spirit Stone," Raven presented the crystal for everyone to take a look at and then began to explain,

"Spirit Stones are the currency for the 'cultivators', it says here. There are three levels of Spirit Stones. Low grade, Medium grade, and High grade. A thousand Low-grade Spirit Stones can be exchanged for a single Medium grade Stone."

"Let me guess; a thousand Medium grade Spirit Stones also equal to a single High-grade Spirit Stone?" Zier interjected, seemingly already having comprehended the entire currency system just from that.