dark sediment in well water

Dark sediment has started showing up in our well water. We have no water treatment or softener system. The sediment is dark and course and looks looks somewhat rusty. We have lived in the home 6 years and the well has been in at least 10 years prior to that. What might be causing this?

Thanks for your help.

Most likely, iron.

A water test would be the best way to find out.