geomagnetic

The story

1. Proto-Hellenic*gaya
2. Ancient Greekγαῖᾰ (gaîă)clip.
3. Ancient Greekγῆ (gê)
4. Ancient Greekγεω- (geō-)
5. Englishgeo-
6. Old Frenchmagnete
7. Ancient GreekΜάγνης (Mágnēs)
8. Ancient Greekμᾰ́γνης (mắgnēs)
9. Latinmagnēs
10. Middle Englishmagnete
11. Englishmagnet
12. Englishmagnetic
13. English todaygeomagnetic

From geo- + magnetic.