Can you post a picture? Give a short description where you found it. How hard it is, can you shine a light through it? Is it magnetic? Ex. I found this rock while climbing a trail through the mountains near Vail Colorado. It is not magnetic. Light does not shine through it. It is heavy for its size. It scratches easily with a common nail. Here are two pictures of different facets. I placed the rock next to an American half dollar for scale. It is black and there are silver mines near the spot I found it.
Metal ores tend to be heavy for their size. Some metals oxidize. Found near silver mines, not magnetic. Black appearance from oxidation of silver. Rock is most probably silver.