What Pokemon evolves from a metal coat on Pokemon diamond?

Well Done for finding one - they're quite rare!

The item you have 'Metal Coat', will successfully evolve an ONIX into a STEELIX. You can catch ONIXs in many caves throughout Sinnoh, but you can specifically find high leveled ones (about Lv55) in the Snowpoint City Temple after beating the Elite Four.

Good Luck!

-ChipChip