There were two possible reasons why this small stone seemed unremarkable: first, because it had become separated from the Stone of Destiny making its magical effect disappear, and second, because he was not a king, so it could not take effect.
Gu Shanhai wasn't particularly covetous of the Stone of Destiny because its effects were limited and were most likely only effective in this world. Additionally, its use was too restrictive. He wouldn't possibly conquer a world to become a king just for such a trinket.
There were simply too many similar things. Why bother with something as troublesome as this one?
The origins of the Stone of Destiny at the church were unclear. There had been speculation on the forums. It was either taken when the family split or found after the division. Otherwise, it couldn't have just appeared out of nowhere.