The other day, I was walking with one of my favorite co-workers (let's call him Ed). Ed started working at our company when I was one month old. He knows his stuff. Ed is very humble, but he is almost always right. I can't tell you how many times I've bought him coffee because I lost a wager to him (engineers tend to make bets about things like thermal coefficient of expansion, porosity, and heat transfer).
The landscaping where we were walking had some areas that were covered with river stone. As we walked and talked, I kept finding pieces of jasper, which I picked up and showed to him. (I've mentioned in other posts that I have a terrible tendency to pick up rocks).
Ed speculated that it was a hard stone because of how shiny and polished looking the jasper was, even in its raw form. I replied that I thought jasper was soft because it is a semi-precious stone, and I seemed to remember that one of the defining characteristics of semi-precious stones is that they are softer than precious stones.
Ed, who has over 40 years of experience working with metal and glass in engineering applications, explained that hard metals are much easier to polish than soft metals. Hard metals also hold their polish better than soft metals. He was wondering how it was possible to polish a semi-precious stone if it is soft. I told him I would do some research on it.
I found a great website about polishing rocks in a tumbler. When you polish mixed rocks in a tumbler the ones with the highest hardness will have the most glossy finish. Also, it is more difficult to polish softer stone; very soft rocks, like limestone, will never develop a glossy finish.
Interesting, but how do rocks compare to metals for hardness? I looked up the hardness of various metals at this helpful site.
Jasper has a Mohs Hardness of 7, which makes it one of the harder semi-precious stones.A steel file, which is an example of a very hard metal, had a hardness of 7 or 8.
Aluminum, which was Ed's example of a metal that does not polish well or keep its polish, has a hardness of 2.5 to 3. That puts it in the same range as limestone (hardness of 3), which is a stone that does not hold a glossy finish.
So, in summary, I pick up a rock and show it to Ed. Ed notes that it is shiny and is able estimate its hardness is the same as hardened steel - and he is exactly on the mark! Meanwhile, I have to look all this stuff up to re-trace what he just knew. I can't believe how well calibrated his engineering sense is!
Ed was right, as usual :)