Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Look for places that landscapers buy rock.
Might be listed as Masonry, Brick, Soil, Stone, Topsoil, Bark or other names.
Bring some baggies with you and a sharpie marker.
When you see some stone you like, take some of the broken bits as samples. Label the baggie.
When you get it home, put each sample in a separate jar of water and test GH, KH, pH and TDS when you start (pure water) and then a few hours after adding the sample, then 24 and 48 hours. If the tests are still changing, run the tests out to a week.
This will give you some idea which materials will react more with your water, and to avoid these.