One of the ancient problems was telling gold from other minerals that looked like gold.
Many people got excited by iron pyrites, which is rock containing a compound of iron
and sulfur that looks like gold. If you had strong acid, it would dissolve the iron pyrites.
I read somewhere that when gold was discovered in California, a military officer named Shermad
who later became a famous general in the Civil War, was consulted because he had previous
experience with gold. He didn't have any acid, but showed that its extreme malleability
was consistent with its being gold.
Gold and other "noble" metals can be dissolved in a mixture of nitric and hydrochloric acid
called "aqua regia".

For a nice step-by-step example of the acid test, rent "The Pawnbroker," one of the late Rod Steiger's best films.

a military officer named Shermad
who later became a famous general in the Civil War,

Yes, General Shermad made his famous March to the Sea where he burned everything in his path 'cause he was sher mad! And some folks in Columbia, SC, to this day, get sher mad when you bring his name up.

Pyrite was also known as "Fool's Gold."

For a nice step-by-step example of the acid test, rent "The Pawnbroker"

And for a quick-and-dirty example, and a wonderful evening too I might add, watch Alec Guinness' Lavendar Hill Mob. :-)

Or, for a real hoot, read The Electric Kool-Aid Acid Test by Tom Wolfe.