Can I fly with a gold bar?

It's allowed, I have personally seen a passenger who booked a seat for a bag. I then asked the passenger and he said it was gold. I was an operating flight attendant.

I also do not recall any rules regarding prohibiting gold onboard, (from an aviation point of view) unless it was in the shape of a knife I assume. Just check with the airline you are flying with in case they have specific rules regarding that (I don't think they would).

Finally, make sure about the customs regulations in the destination country, that's what I would worry about.

Anecdotally, you can buy gold at a vending machine in Abu Dhabi airport, in the outgoing terminal. Whilst policies at destinations will vary of course, it's highly unlikely to be widely illegal whilst being sold at an airport.

Gold ATMs are available across the world - not sure I would have believed it if I hadn't seen it myself while transiting through Abu Dhabi! :)

You are carrying a half-million dollars in gold on your person and you are flying into... Russia? Keep a sharp eye out.

As Nate points out, you may have to pay an import duty in Russia, but apparently there are no restrictions on the British end. In the US, you would have to declare it. Last year, I got into a nasty run-in with ICE who were convinced I was exporting currency.

My research corresponds with yours: nothing about carrying metal on the plane. I imagine getting through the security checkpoint is going to be a hassle though. Gold is pretty impervious to x-rays.

Not exactly your destination, but Werner Rydl was arrested in Brazil when he was carrying a bar of gold.

Also others have been arrested when they were unable to promptly provide the origin of the gold they were carrying.

Since you said in some comment that you will just transport the gold but you're not the owner of it... You'd be a mule, or, if you were in Brazil, mula.