There are glowing gems sticking out of the ground, walls, and stalactites all over the Underdark. You can smash them and get a bunch of these little Purple Fluorite Shards. I found the best way to mine them is by using Eldritch Blast, since it can reach all the hard to get to places and the stones are vulnerable to Force damage.
I think I got about every one possible in the Underdark, and I ended up with around 500 of the shards.
As you can see, they're worth a lot of coin!
You, too, can live the high life of an Eldritch Blast miner. All in the beauty and safety of the peaceful Underdark.