Yoma 44b - seven golds and metals of the ancient world

"Rav Chisda says, There are seven types of gold: gold, good gold, Ophir gold, paz, shachut gold, "closed" gold, parvayim gold..."

The Maharsha has a fascinating discussion of these, where he says that these seven golds are alloys of gold and one of the "seven metals that the scientists know". (One of them, of course, is just gold.) I wondered what those seven metals could be: the Torah just lists six, in the discussion of hag'alas keilim after the war of Midyan
gold, silver, copper, iron, tin, and lead.

I looked online and found that the "Seven Metals of the Ancient World" is actually well-known. So what is the seventh? It's mercury. Mercury, of course, is a liquid, and so you can't make vessels out of it (except temporarily at about 38 degrees below zero). It actually makes a good alloy with gold; it's called amalgam, and sometimes has been used in dentistry.

I also wondered about bronze and brass. They have been known for thousands of years; the time of Matan Torah was in what is called the Bronze Age. Bronze is copper and some tin - tin is on my list already, and bronze is probably also called נחושת in Tanach. Goliath was said to have armor of נחושת, which must have meant bronze - far harder and tougher than pure copper.

Brass is an alloy of copper and zinc - so why isn't zinc listed? But it turns out that zinc was not known as a separate metal until the 12th century; brass was made by smelting ore that contained both copper and zinc together. Maybe it was called נחושת as well: if there were seven varieties of gold, there could be many varieties of copper - see the gemara about the copper in Nikanor's gates (36a), and see Ezra 8(27).