Wow.  When we put the formula shown here into Google, the results come back all mixed up.  The official agent (and the card itself, but only in Korean) calls this Limonite (갈철광), which either does or does not have a different chemical formula.  Are you a geologist?  The pre-printed stamp is large and very similar to the front.  Possibly not a lot of postcards in the world are quite like this one.  Grade: 1