I came across some black gravel I screened out for the color, but running a magnet found out it has iron in it. Now, question, is there any reason as to why a person can not use this , have heard that metal should not be used, but there is metal on the wheels, of rolling stock, locomotives, and the knuckle couplers have metal, in the meantime I will await for your answer before putting this stuff down
The reason you don't want to have any metal parts in your ballast is because they could magnetically get pulled into the motor or running gear of your locomotive. And getting metal caught up inside moving parts would be bad.
My guess is it also may be too big. A one inch rock in HO is not very big. It would be a lot smaller in N scale.
Yes, don't use anything magnetic for the reasons already stated. Just as an FYI, the stones used for main line ballast average about the size of a man's palm. So consider the size of the hand of an HO scale figure when looking to get ballast--it's smaller than you think. Secondary lines, yards, sidings, etc. often uses cinder fill, which is even smaller.