Certain types of currency are absolutely unforgeable or easily detected if they are forged. Gold, for example. It's how we discovered specific gravity.

What made gold a currency was not the metal, it was the stamp of the local ruler underwriting the claimed value.

The metal was just a convenient pick because it was useless for tool and weapon making, and didn't degrade quickly if left in some damp place.

Low melting point also makes it relatively easy to re-form and test for purity if that becomes necessary.

Nonsense. The average person, me included, could not detect impure gold if given a piece of it.

It's not that hard. Take a Black Rock, and a few samples ok known purity, 75,80,95,100%. Then scratch the gold sample across a set of lines, of these purities, and see which is the closest.

You failed.

A lot of counterfeit gold has insertion of other metals in the center (tungsten for example which has a similar density)

https://static6.businessinsider.com/image/505a1d2eeab8ea8610...

Note that I specifically talked about density, not weight.

Wikipedia: Because the density is so similar to that of gold (tungsten is only 0.36% less dense), tungsten can also be used in counterfeiting of gold bars, such as by plating a tungsten bar with gold

That wouldn't pass the scratch test mentioned above. The gold plating would wear off (and gold doesn't hold well to tungsten, usually requiring an intermediary layer like titanium or copper.)

Because every till assistant was listening in physics class and knows how to calculate the specific gravity of that ring people want to barter with in an emergency.

Hey, current till assistants think $2 bills are fraudulent.

I see you're on the market for tungsten...