What you're discussing is a test for hardness. The standard is Mohs scale of hardness. All this test would tell you is whether your metal coin is harder or softer than the rock in question.

I know a bit about ancient Greek and Roman coins. You can find coins that have been clipped on the sides (the ridges on US quarters now are a relic of a method to prevent this practice). You also find coins that have a gash cut into them.

Clipping, or shaving, could be used to verify the metal, but more commonly used to remove metal for later reuse. If I shave a bit of metal off a coin, and then buy lunch with it from you, and you don't notice it's shaved, I've just made a bit of money.

The gash is almost certainly to ensure the coin isn't some base metal plated with silver or something.

But the fact is authenticity wasn't exactly a consideration: metal content was in most circumstances. There are tons of different coinages in the ancient world. Many Greek cities minted their own coinages. Even individuals like Hannibal minted his own coinage. They come in all kinds of denominations and standards. If I remember right, an Attic drachm contained less silver than a Rhodian one. The Ptolemies purposely struck their coinage underweight, and required that only their money be spent in Egypt, while converting foreign currency at 1:1, thus gaining a lot of silver in the process.

The general rule is "bad" coins (coins with lesser metal content) pushes out "good," meaning if you have a choice between spending a lighter coin or a heavier coin, you spend the lighter and save the heavier. You don't care about what is "authentic" except as far as one mint might be trusted for a consistent metal content of their coinage.

You see governments from time to time try to manipulate this by debasing their coinage. Rome, for instance, had debased their coinage so much that by Diocletian they had a serious inflation problem on their hands. Diocletian tried to control rising prices by law, which of course failed. It took Constantine to rework the coinage system and re-establish trust in Roman coinage.

There are some interesting outliers. I've already mentioned the Ptolemaic coinage, which was a sort of fiat currency. Pergamum also had a similar fiat system.

The weirdest coinage I know of is from the period following the 1st Punic War. Carthage used a lot of mercenaries in that war, and after paying the indemnity Rome imposed at the end of the war, ended up with no money. But they still have all these mercenaries sitting around who, you know, want to get paid. Carthage starts moving these mercenaries out of the area claiming they'll be paid out, you know, "over there."

When the mercs figured out they weren't getting paid, they revolted, and many of Carthage's Libyan allies revolted as well.

Now, no one has any money, but you still need money to run a war. The story we have from Polybius is that the Libyan women donated their copper jewelry to the war effort, and it was melted down for coinage.

That seems bizarre, because if your unit of account is something silver like drachm or a shekel, you need a hell of a lot of copper to have any significant amount of money. About 10 times as much copper.

In the 40s, Robinson published a hoard of coins he connected to this Libyan war. Another such hoard was found in the 50s. Sure enough, these are bronze (so, copper and tin), but mixed with arsenic. I've handled them. You can't touch them without gloves because of the poison. Thing is, that arsenic gives the bronze the appearance of silver. Someone valued these enough to leave two hoards behind.

Well, that was a bit rambling. My point is authenticity wasn't as important as metal content, with some outstanding exceptions.

If you're interested in this stuff, Hawgego, Ancient History from Coins, is an excellent primer, and it is (or used to be) required reading for students at the American Numismatic Society.