I
could give it thee. I didn't remember peridot, it's (Mg, Fe)
2SiO
4
So it qualifies according to my sloppy question.
But I won't, because I didn't see it and there's a better answer.
The one I had in mind is MARCASITE, which is FeS
2
I should have said the ONLY metal is iron, then you'd all be wrong except one.
That site includes Minerals and lots with no iron or only a bit.. We can argue the difference... not all minerals are gemstones for sure.
Iron can be in just about anything as an impurity.
But the winner - probably by accident, is:
Fools gold = Iron pyrite is Fe 2s, which comes in two possible crystal structures, one orthorhomic, the other cubic, if you remember your ball and stick models.
The less common orthorhombic one is used as a gem, white iron pyrite, called marcasite. It does sparkle when cut and polished.
View attachment 280323
But does suffer from oxidation...
A more bizarre wrong answer would have been Cut Steel, which used to be used 18th-19th C ish.
They used to take piano wire, grind it to have facets, and case-harden the heck out of it. The lustre changed according to what they put in the case hardening along with the carbon. Then, hard as hell, it would take a very high polish.
View attachment 280324
View attachment 280326
Not too many good examples remain, as they tend to rust away.
If you look at the back of whatever it's on, you can see.
So there you go
@OddsBodkin , it's yours.
