It could be a lot of things including Arsenic!!! Mercury will amalgamate ( coat ) with Gold, Silver, and Copper.
DO NOT HEAT OR USE ACID on what you have because your question leads me to believe you may not know all the safety precautions as the fumes can be deadly!
If you have a lot of this material, send it to a Lab for testing or take it to chapter meeting and find a member that has the experience to help you. Ron