One of the reasons gold became a store of value was the ease in assaying it, that is, determining its purity.
Gold is usually alloyed with another metal because in its pure form it’s very soft. For example, 24K gold usually contains just a touch of copper to keep you from being able to bend the gold with your hands. 18K is usually more common for gold objects because it has a better wear characteristics. However, for coins or bullion, you want it about 99.99% pure.
Now, you can assay gold by melting it (gold is so dense the molten gold falls to the bottom) but that’s expensive and time consuming. Its fine if you’re running a mint to purify gold in this manner (and we still do that today when we take broken jewelry to turn into bullion), but in most cases you don’t want to take the time or energy.
This is where this comes in
https://www.mineralminers.com/images/tourmaline-black/crystals/tbkm654-black-tourmaline-crystal-specimen.jpg
This is black tourmaline, a mineral made up of boron, silicon and maybe some aluminum, iron, magnesium, sodium and lithium. It’s pretty on its own, but the ancients found it had a much more valuable property
https://qph.cf2.quoracdn.net/main-qimg-62b5c4d0ffd790c8c1e56f22eb7724d1-pjlq
When you rub a piece of gold against it, the colour of the scratch correlates with the purity of the gold. Cheap, instant, irrefutable and mostly non-destructive.