we can assume that the ghost bloods gave him gems to soulcast with, the reason for this is so other people can see that Lin was making money in a legal way (finding veins of semi-valuable stone) the ghost bloods wanted to have Lin make a play for high prince and it would be harder for him if people knew that he had just been given a bunch of money for a group like the ghost bloods but if he was mining stone they would see him as truly wealthy and would give him more consideration for high prince.