The profound stones are the crystallisation of pure battle energy that are mined from quarries that can be used to increase one's cultivation when absorbed.
Because of their use and resourcefulness, they had replaced other forms of currency and one needed these profound stones to buy anything in this world.
Thanks to the previous owner, Yang Chen was aware that there existed several grades of profound stones. Bronze Profound Stones is the lowest grade and is the most common currency that was used by everybody in this world.
It can basically buy you all your needs, whether it be clothes, food or drink. However, when it came to wants like, battle skills, elixirs, pills and all, one needed higher grades of profound stones for the transaction.
The previous owner of this body never came across anything higher than bronze profound stones thus he was mostly unaware of what other grades were there.