Get some of the littlest pieces of whatever you like. The broken off bits.
Test those bits in glasses of the water you use in the tank. Tap? RO? Do you add peat moss?
Test GH, KH, pH, TDS for a week or two.
If any rocks make changes you do not want in the tank, then do not use that one.
I have some that your store calls 'Imagination'. Red lava in weird shapes. They make interesting caves.