Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Ditto: Those look like rocks that dissolve easily, such as Holey Rock, Limestone and related materials.
I would test these in a bucket of water, testing GH, KH and pH (TDS if you have a meter) every few days for a week, then weekly for as long as you want to run out the test.
I think you will find they would make good rocks for a hard water tank (live bearers, African Cichlids, certain Rainbows) or a brackish water tank.