Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I do not recognize it specifically, but it sort of looks like a type of basalt that is used locally in the landscape.
Generally igneous rock is aquarium safe. The high temperature of its formation locks all the minerals in so they do not come out in the water.
You can sure test it, of course.
Put some in a jar of water and test right away, then every few days for a week or two.
GH, KH, pH, TDS will be the most telling. If you have other tests like iron or phosphate that could help.