My dm conveniently just gave us 1m worth of diamond dust thinking it would be useless 

My main issue is that it's the EXACT same cost as Greater Restoration, which is best known for Coffee-Lock characters avoiding exhaustion levels, removing Curses, and other HP threatening effects. Keeping the concentration is not the issue, unless you are casting it on yourself; but the cost for what you get is ridiculous. If you are going to consume the material component, then don't make it 100 gp worth of diamond dust; that requires you to not only have a diamond worth 100 gp, but also have the ability to grind said diamond into dust. If you also have a Coffee-Lock in the party, the DM may limit the number of diamonds that you can even find, making Greater Restoration the primary reason that you collect the diamonds. I do agree with the people that intend to use this as an ally buff, the concentration is important, but the spell cost makes this not worth it in my opinion. As a concentration spell, you are already preventing people from spamming it over the whole party, and at lower levels (when you will most likely need/use it), you only get 1-2 uses of it per day. As stated by someone else, resistances can be acquired in many ways, so a forth level spell with this kind of cost... not really worth it, but that's just my opinion.

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Quote from FeyPhoenix >>

100g worth to cast a concentration spell that only lasts an hour, and doesn't protect against magical attacks... seems under-powered to have such a high cost to me?

Exactly what I thought. Prohibitively expensive in both monetary and taking the one concentration you can manage. Borders on useless given how many times my wizard has been attacked with non-spell or non-magical weapon attacks.