There are many possibilities. There are also two kinds of material here.
A geode is an outer husk of one rock with an inner core of another substance. Really, that is a simplification but accurate enough for this discussion.
The outer portion or husk could be a number of substances. It could be slag, which is waste metal. That would be magnetic. If it were a meteorite and that husk were a burnt husk from entry into the atmosphere, that would be cool, too.
I suspect it is a porous rock formed by volcanic activity which cooled. As it cooled, air bubbles formed and over the years, sediments permeated the husk and a concretion of sand and trace elements formed the inner rock.
The outside rock could be basalt, flint or even hematite.
Flint would be hard and scratching it with a nail would not be possible. It would also spark if you struck it together with steel. It would not be magnetic. Hematite would be, most meteorites would be, too. And, slag. Lots of questions for you but some answers, too. The answers will serve to narrow the choices down so we can complete our identification