Nouns ending in -er are usually masculine.
Is fer masculine or feminine?
lefer
The rule behind it
94.6% of -er nouns are masculine
In context
L'or est plus lourd que le fer.
Gold is heavier than iron.
L'or est plus précieux que le fer.
Gold is more precious than iron.
Le fer est plus dur que l'or.
Iron is harder than gold.