Not the rarest mineral in the universe, there are still entire planets that lack this valuable mineral. But not Earth. Earth is practically made of the stuff. If you intend to keep humans for any length of time, you'll want to remember that they need iron in their diet. It is an important component of their blood. Without enough iron, humans become weak and useless. You want to be careful not to give them too much! It is a delicate balance. They do tend to react to its lack or over abundance quickly, so an attentive owner will learn to recognize the signs of an iron imbalance.

It is easy to mine iron on Earth, however as an interdicted planet, mining is forbidden. Fortunately, it is available almost anywhere, so if you can get to a remote location - some suggest starting on the sea floor where humans are scarce - you can quickly fill your hold with the stuff and get away before being discovered by the regulatory enforcers or the humans.

Is mining iron worth the risk?

— space pirate second-in-command contemplating an empty TREIN car

Be aware that iron is heavy, so you'll need extra boosters to get it out of Earth's gravity well.

Get away with it, and you'll have a valuable haul that can easily be sold for use in building materials for ships and space stations, used as a nutrient, turned into tools, or sold as a raw material. One good haul can keep you in credits for years. And no one needs to know that you broke the interdiction to get it!

Do you know how much a full load of that stuff would be worth?

— space pirate captain