a level that just churns out gemstones below the deep deep extremely high level. add diamonds found down there as the exclusive one. make the others have an equivalent rate to ores on other levels.
I don't think this would be as helpful and cool as it sounds. The gems sell for 4k so for those of us with an expanded mining cart (at max: 30 slots), we'd be making 120k pebbles per visit down there. That is a lot of pebbles to affect the market with, and it would devalue pebbles for newer players even more.