looking at the FAQ, I believe I am missing this one, but it's in Japanese
10. Balnaul Highway, northeast section バルナウル街道 南東部 On the western side of the map, there should be an area where you can jump down. There should be a hole you can crawl through in the cliff near where you jump down. A treasure chest inside holds the orb.
I have no idea what Balnaul Highway could possibly be. My thought was that is Barnaer Highroad as that's the closest english location to that road, but it says the mysterious jewel is in the northeast section, and Barnaer Highroad doesn't have a northeast section, nor does anywhere else in Barnaer highroad match the description of where to find that jewel. Where is this place?
"The easiest way to stop piracy is ... by giving those people a service that's better than what they're receiving from the pirates." ~ Gabe Newell.