磁石是否永遠都有磁力？

磁石有兩種類，天然礦物永久磁石永遠都有磁力而人造磁石縱有一天會失去磁力。

The magnet will loss its magnetic property, but not due to oxidation.
The magnetic property comes from the alignment of the atoms. Every atom is a small magnet, if they are align randomly, you will see no magnetic property. If they align well, then you will see its magnetic property.
But the alignment will relax as time pass, if you watch the dramma on TVB, you may heard that there is a temperature, the Curie temperature. It is about 500 degree C for iron, which is material dependent. If the temperature is higher than it, the atoms will align randomly and lost the magnetic property.

磁石採集後放置日久，發生氧化，其磁性便會減退，乃至失去吸鐵能力。