In the movie Fantastic Beasts and Where to Find Them (2016) the first critter we are introduced to is the Niffler. I am confused with its exact nature. Is he attracted to all metals or to precious metals or to anything that is glittery?
It seems it is attracted to any glittery object.
Nifflers are gentle by nature and could even be affectionate towards their owners. However, they could destroy belongings looking for sparkly objects, and for that reason it was inadvisable to keep them as a house pet.
Same is mentioned at PotterMoreSite
Long-snouted, burrowing creatures native to Britain with a penchant for anything shiny
They were first introduced when Harry was in his fourth year at Hogwarts (emphasis mine):
"These're nifflers," said Hagrid, when the class had gathered around. "Yeh find 'em down mines mostly. They like sparkly stuff… There yeh go, look."
One of the nifflers had suddenly leapt up and attempted to bite Pansy Parkinson's watch off her wrist.
- Harry Potter and the Goblet of Fire, Chapter 27 "Padfoot Returns"
So this confirms what the other answer already states.