The way this works is the gold jewelry is rubbed against a black slab rock and some of the gold rubs onto the slab. Next we pour an acid onto the marking and watch the magic happen. If the color stays this means we have real gold. If the color vanishes and the acid removes the marking we have fake gold!
Now we have different acids for each of the different karats of gold, and this is how we test the gold !
Also, we can use magnets to test gold as well.... because gold is not magnetic !