[RPG] the weight of diamond dust

No, Major Creation works like Minor Creation, which states:

Attempting to use any created object as a material component causes the spell to fail.

You can create your diamond dust, but that dust cannot be used as material component for spells. You could probably attempt to sell it, make some money, and then use it to buy non-magical diamond dust. But be careful to not get caught.

Unless the backpack becomes a creature unto itself (via something like animate objects) it does not have a 'maximum' weight. Mostly because going down to that granular a level for random equipment is generally not worth the effort it would take. And, as you, yourself, pointed out, different materials can weigh different amounts, so there is no "standard" definition of how much a cubic foot of material weighs.

Carrying capacity is only affected by magical items, like muleback cords or a bag of holding (which only has an effect in that the things inside it weigh nothing in regards to your carrying capacity), or a masterwork backpack (which raises your effective str score by 1 when calculating carrying capacity). Containers such as barrels, waterskins, backpacks, etc... are more of a convenience for describing how your character is carrying all the random stuff adventurers carry.

As a gm, its far too much work and nowhere near fun enough for me to keep track of the size of everything to make sure it could fit in your various packs as long as it doesn't go over your carrying capacity. Also as a GM, I'd be insane or a giant pain to force my players to keep track of that as well. Saying that, though, its all about what I would deem as reasonable, you could probably throw 1000 coins, some pots and pans, food, and whatnot in a bag and I won't question it, but if you try and fit a 3.5 ft long sword in the bag and not have it be easy to steal, or trying to fit a small cursed item that weighs 1000 lbs in your bag without it ripping I'm gonna question you.

Essentially, its on a GM by GM basis.