Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Look in rock yards that sell in bulk and bags to landscapers, masons and other trades.
Real rock will hold its color. Coated rock will not.
There are 2 gravel to pebble sized materials that are black or nearly black found near me.
One is a bit large as a substrate, but is a great accent. That is Mexican Beach Pebbles.
The other is a gravel called Sage Creek. Note that each store may call the rock something different. Go get samples. Test for GH, KH, pH and TDS changes in the water. These types of stores do not mind you taking a little baggie of rock home to test. Bring a sharpie, too, so you can label the baggies, just in case there are more than one rock you like.