The Last Remnant

You need to harvest Necrotic Metal from the 3rd or 7th path. These can only appear on the map very late in the game. After Underwalt appears.

To open the 3rd path you need to talk to a Quitsi near the counter in the Underwalt guild (note: DO NOT enter underwalt until you have completed the side-quest "The Fallen" or you'll miss a 140g achievement and the chance at 2 x 200g achievements later in the game!).

Head into the third path, go through the door, take the first right, then the first left, then drop down a ramp and walk forward a bit. The dig spot is on a rock in front of you and produces Blacksteel Ore normally, but if Mr. Diggs triggers a rare item it will be Necrotic metal. Just mash the A button until Mr. Diggs comes up with the goods :)

7th path is opened a bit later during the story. It is accessed via the 6th path. Not sure where the dig spot is in the 7th path.