Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I agree that whatever is cheap and local is probably the material used by the RR.
On the other hand, landscape rock is hauled by railroad across the nation.
If the RR had a specific need for rock with particular characteristics, they would certainly be in the position of hauling whatever they needed from wherever to wherever.
Go to the landscape stores, you are likely to find the widest selection of rocks. Bring baggies and a Sharpie marker to label samples.
Look around locally, where it is legal to collect.
Take your finds home and test:
A glass of water with nothing in it, just the water.
A separate container for each sample.
Test GH, KH, pH, and TDS at the beginning and every few days.
Carry the test out as long as you want, but a week is reasonable.