There are companies which are specialized in producing devices which let you determine the composition of material without destroying the actual material. A popular method is to send X-Rays in the direction of the material and then measure the angle of the reflected X-Rays. Every material has a different angle of reflection. If you want to know how those devices look like go to:

http://www.bruker.com/products/x-ray-diffraction-and-element...

They are very expensive but they are getting cheaper and smaller every year but those methods are far away from being simple.

I have always wondered about this: are you able, with simple methods and no melting, to guarantee that a gold bar is a gold bar, with a 99,99% certainty?

By producing a gold-coated bar:

- density could be faked by putting something else in the inside of the bar

- nitric acid test would not work, since the surface is gold

- hardness would not work, if the surface layer is thick enough.

- no idea about magnetism

- other tests?

Gold is soft, so you can probably push a needle inside to check for a tungsten core (most common way to fake gold bars)

Not as fancy as X-ray techniques, but much cheaper and readily available

Maybe you can analyse the propagation of mechanical waves within, not sure (so a pure gold bar will 'sound' differently when struck with a soft mallet)

Yes, X-ray fluorescence. Modern handheld devices are fast, accurate, and relatively inexpensive, see my other post.

Might be worth noting that the things denser than gold are probably all going to be more expensive as well, so there isn't really any way to hollow out your gold and fill it with weights to correct the density. Even DU falls short of being dense enough todo that.

Tungsten has a density extremely similar to that of gold (19.25 g/cm³ for tungsten, 19.30 g/cm³ for gold), and is substantially less expensive. And tungsten counterfeiting does happen: http://www.zerohedge.com/news/tungsten-filled-10-oz-gold-bar...

Detectable if you measure density carefully enough (though do people? I don't know). If you did that with plutonium instead (way more expensive than gold obviously, so it would be a silly thing to do) then you could get it as close as you wanted.

As an interesting sidebar, there's a series of photos of some of the scientists on Tinian posing with a small box containing the core of the Fat Man bomb - a 3.5" diameter sphere of Plutonium that was apparently worth at the time the equivalent of about $5 billion dollars.

http://nuclearsecrecy.com/blog/2011/11/25/friday-image-posin...

It's certainly detectable with precise instruments, but it's not practical to test each and every bit of gold one acquires so thoroughly. Hence the opportunity for counterfeit gold that passes casual inspection and weighs about the right amount.

And DU would be ...?

EDIT: I think you meant Tungsten, also known as Wolfram (symbol W): http://en.wikipedia.org/wiki/Tungsten

Gold: 19.30 g/cm3 (http://en.wikipedia.org/wiki/Gold)

Wolfram: 19.25 g/cm3 (http://en.wikipedia.org/wiki/Tungsten)

EDIT2 (I can not reply to your reply): So you meant Depleted Uranium (http://en.wikipedia.org/wiki/Depleted_uranium)

Depleted Uranium: 19.1 g/cm3

So Wolfram is a better substitute for Gold, in terms of density. I do not know about prices or other physical properties.

Pretty sure they meant Depleted Uranium by DU. http://en.wikipedia.org/wiki/Depleted_uranium

Has a lower density than Tungsten, though so would make a poor counterfeiting material in both cost and physical properties.

Sorry yes, depleted uranium. Tugnsten also comes close, but not close enough to get it perfect. If you can accurately measure density you could still detect that.

The question I suppose is does anybody measure density carefully enough to detect tungsten, and if not, should they be?

Yeah that's already happening. Look up gold plated tungsten.

Modern technology uses X-ray fluorescence. You can literally just point a handheld device at a block and find out what it's made out of (including Gold). You can do the same with jewelry and determine if it's made out of solid gold, gold plate, etc.

Here's a product video of one example: http://www.youtube.com/watch?v=1Fu8uLBdgcE

Here's another neat video demo: http://www.youtube.com/watch?v=rYLxmRsFIWE

Edit: Also, the Curiosity rover has an X-ray fluorescence instrument as part of its "CheMin" spectrometer which it uses for analyzing the composition of Martian minerals.

Eureka.