Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Scratch the rocks with different things.
If you can make a mark when you scratch them with something soft, (a bit of copper wire such as the solid wire found in Romex) they may not be so good in the tank.
If you can only make a mark when you scratch them with something hard (steel nail), they are a better choice.
Then pour a bit of acid (vinegar, lemon juice, or stronger acid) in the scratched part. Does it bubble? Not so good for a soft water tank, but might still be OK in a hard water tank.
Put them in a bucket with some water from the tank, or prepared water the way you will make it for the tank.
Test the GH, KH, pH, TDS when you start, and every few days for a week.
If there are changes then these rocks are probably not good for the tank.
They look to me like they will be OK, but I would still test them before adding them.