In BDSP's grand underground, are there any methods as to increase how my mysterious shards I find? This can be anything to do with digging more efficiently to finding better spawns or anything else.
While there is no way to officially speed up the process, there are a couple of tips to try and make the process of finding them more efficient.
A little side note: Some people think that entering the Grand Underground from near Ramanas Park yields more Mysterious Shards. However, this is unverified, so take it with a grain of salt. These 2 tips will help make your process of mining more efficient, thus helping you to find more Mysterious Shards over time.
Source: https://www.thegamer.com/pokemon-brilliant-diamond-shining-pearl-bdsp-mysterious-shards-location-guide/
https://dotesports.com/pokemon/news/how-to-get-mysterious-shard-s-and-l-in-pokemon-brilliant-diamond-and-shining-pearl