Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Here is a test:
Test the GH, KH, pH and TDS of some water.
Add some of these rocks to the water (perhaps in a 5 gallon bucket)
Test GH, KH, pH and TDS of the water every few days for at least a week, longer is better.
Test with any other tests you have, too. Such as iron.
If there are no changes then the rock is probably OK.
That intense red-brown color makes me think of iron ore, though. If it looks like it is rusting (especially once it has been under water a few days) then it probably is.