You can only get the rare material from resource nodes, not from chests.
As for ability kills, this will depend on your class. For example, if you are a Titan and got a void sword, you have no offensive super as a defender to use for ability kills. But you can use things like montecarlo and armamentarium to boost your grenade/melee charges for the farm.
Any mission where you can get a group of hive thrall running at you is a great farm option for group grenade clear out
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From the last 2 materials I've gotten, it looks like you get 1 material every 13 nodes (I counted, because there's nothing else to do), which makes it 130 spinmetal nodes total. Still seems a little excessive to me. And as far as things to make the grind easier, they're all also things you have to grind for or be incredibly lucky to get as a drop. So that's great if you already have them, but you're in for even more grinding if you don't.
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You must not have picked void edge then come back here and complain about the materials for that (Relic iron). And for Spin metal I went to skywatch small map easy to see just equip a sniper with a spin metal ghost