What... what is this spell even for?!

I'd use it to replicate a key I've seen, sell some fake jewelry, duplicate currency, manifest silver weapons, 

Those are actually decent uses, as long as you aren't planning to hang around that town too long, making a weapon would be good for probably 1 or 2 fights, since it would only last an hour being made of a precious metal like silver. Gotta be really careful of the jewelry too since it would only last 10 minutes, that's not long to barter, sell, and then vacate their area before the jewelry up and vanishes. I don't have a very good thief/ rogue brain so none of those things occurred to me really.

Pretty expensive spell slot for such simple uses though. I suppose you could probably duplicate a set of thieve's tools for your rogue too, if theirs break suddenly and you desperately need to pick a lock to get away.

Has no-one else thought of creating a 5x5x5 foot cube of solid gold over someone that can't or won't move for a minute?

Why gold when stone is everywhere and lasts longer?

Oh even better, make it out of candy. Death by Candy Crush.

Why use gold when you can use lead?

Quote from Fattest_ace >>

Has no-one else thought of creating a 5x5x5 foot cube of solid gold over someone that can't or won't move for a minute?

 I have -- as a DM I wouldn't let you do this do a creature, d/t RAF, but the gold cube has MANY potential uses.

The mass of the 5-foot gold cube is 125 tons; if you can spare a 6th-level spell slot, it's 1,000 tons, which is a nice round number. So let's use a 6th-level cube as our example.

A 100sq ft object weighing a thousand tons would shatter any ship, crashing through the decks like a barbell through wet tissue. It would bring down any tower, shatter any wall. Put it in the middle of a bridge -- whoops, no more bridge. Are you in the middle of a big siege, and getting concerned about the sappers approaching your wall? Their tunnel isn't going to hold up to an additional 1,000 tons of weight.

Quote from Gbellis >>

Why gold when stone is everywhere and lasts longer?

Oh even better, make it out of candy. Death by Candy Crush.

 Because of the density. The density of gold is .6 tons per square foot. That's denser than lead. The density of granite is barely a tenth of that.

Or, create expensive material components for spells? Most have a casting time faster than 10 minutes.

Can't be done sadly

"Using any material created by this spell as another spell's material component causes that spell to fail"

Darn. Should've guessed the rule writers would forsee that happening. lol

Could you use this spell to create multiple of the same things of it does not surpass the size restriction?

i.e. 5x5 ft worth of ball bearings or arrows?

Could you use it to create Plate Mail?

It says in the spell text that using anything created with this spell as materials for another spell causes that second spell to fail.

Quote from doc_ketamine >>

Could you use it to create Plate Mail?

Yes - the only question would be from a duration perspective.  If you chose Gold or another precious metal that itself is stipulated (1 hour).  For any other kind of metal e.g. Steel then there is probably a ruling you'd need to agree with your DM.  I'd say steel would be comparable to crystal from a value perspective and 12 hours would be fair.

Why is the only 'Make a thing' spell in the game an Illusion spell and not Conjuration? You make a thing out of nothing! How is that not Conjuration!?!

I feel like this would be a great spell for some kind of rogue multi-class.  Create a temporary forgery for a heist, conjure a temporary pile of gold to procure a valuable item, etc...

Fabricate - Forge domain Cleric Spell

Make anything provided you have the materials and tool proficiencies to make it normally and BOOM

What would take a Smith months or a year to make in game and you instantly make a full suit of plate armor

Correct me if I'm wrong but I think the weight is a bit off. A quick google search and calculation using the first result I got gave me this: a 5ft cube of solid gold should weight about 75 tons and a 10 ft cube about 600. This might, of course, be wrong, I'm not sure if I calculated things correctly.
Don't get me wrong, this is still the weight of at least 3 full grown blue whales or about 430 average weight cars compacted into a 10ft cube, it should still be able to crush pretty much anything. But if the DM decides to look it up, it would be confusing if he got different information and it might lead to small and unnecessary conflicts around the table.

Either way, I'm totally going to use this, I love the idea of just crushing my dm's dreams and plans under a 10 ft cube of gold (or platinum, I think it weighs more IIRC).

Also, would you allow someone to completely encase a creature in the material created? would be a fun way to just remove someone from the fight for an hour or so.

I did not spend much time on this calculation so it could very well be wrong, and if anyone who knows this stuff wants to do a proper one that would be great. 

For uses, one immediately useful use could be derived from that exact thing: The matter cannot be used for spells.

Boom. The BBEG's plans can be ruined by just... replacing their items with items made by creation. They try to cast their enormous spell that ends the world, and... nothing happens. They try to cast a spell that requires something irreplaceable, and nothing.

This is on top of the other uses for creation. Everything minor illusion can do, but on a much larger scale because what you're making is actually real and tangible.