@ShionSinX:
1. Please read chapter 24 again.
On the very first page Ryouta and Emily have the following conversation when they leave the town Cyclo and see nothing but a wasteland without even plants.
Ryouta: "...It really doesn't matter how many times I look outside Cyclo, still looks like this."
Emily: "The only thing that doesn't become a haguremono is air.... water ....and earth after all. Except for places where people gather such as towns and dungeons, everything is like this."
Ryouta: "I see... But even though .... There is really nothing... To think outside the city is a wasteland. This world is really strange.....!!"
So there are no things like mineable ore outside cities and dungeons and hence you cannot mine such gems except as drops from dungeons and haguremono.
EVERYTHING in that world is supposedly originally some kind of drop from a dungeon.
That is why Ryouta was sure that the ring he bought would turn into a haguremono.
Of course, this kind of setting would make the existence of that world impossible to begin with but that is the setting the author has decided on.
2. My "plan B" suggestion was that he should simply buy another few of those rings from that merchant, turn them into haguremonos, beat them, get the higher drops, turn those higher drops into haguremonos again, beat them again, get even better drops .... repeat that till you have an absolutely OP item.