The best way to find treasure in the underground is just to keep grinding, friend. And follow the guide shared by MonkeyBusiness in the comment above for shards in the overworld.
An easy way to grind these, back in the day I played Platinum, was bury a metric ton of spheres and start a sphere farm in the ground around your base. As they grow, treasure grows in the walls, increasing your chances of successful digs. But it takes a bit for them to grow, unfortunately.
I wish it was that easy in the remakes, but they removed that farming function like all the rest of the cool parts of Platinum.
