Working on a home brew adventure for DH where a team of acolytes is sent to a Fedual World to investigate a disruption of the world's Imperial tithe. Was thinking of something like Adamantine but I'm unsure if a society that had just discovered black powder would be able to mine it.

Any suggestions?

Well just because they might not have had the skills to mine it, doesn't mean that either the Admech or "whoever found it" doesn't have the skills and tech brought in to do it. Might be one of the 2 places on planet where you could "find" tech goodies the other being the Palace.