Polished (or How Engineers Speak in Private)

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 :)

Why Does Labradorite Flash?

We all have our favorite stones; mine is labradorite. Labradorite is a transluscent, gray stone. What makes it special is the flash of color - viewed at the right angle, you can see flashes of bright aqua blue, green, yellow or orange. You can see the flash in these labradorite cubes I bought a few months ago.

The flash is beautiful but why does it happen?

This is where my nerdy, engineering background is going to show through. I did some reading on mineralogy. I found out that the flash is called "Schiller effect". To understand Schiller effect, you need to be able to visualize how labradorite forms.

Labradorite is in a family of minerals called feldspars, which are classified by their chemical composition. Feldspars have a crystal lattice made up of silicon, aluminum and oxygen. In addition, depending on the type of feldspar, the crystal may contain potassium, calcium, sodium, or a mixture of calcium and sodium. Over 50% of the rock on Earth is feldspar.

The reason we aren't surrounded by gemstones is that most of these rocks cooled quickly. The molecules in the rocks are most stable chemically when they are arranged in large crystals, but the rapid cooling caused them to frozen in place before they could arrange themselves. Thus most rocks are made of a mishmash of microscopically small crystals. When feldspars cool slowly - for example the molten magma is trapped underground - there is more time for the molecules to arrange themselves into large crystals before they are locked into place.

Now here is the really interesting part: when a mixture transforms from a liquid to a solid, the result can be... complicated. The first crystals to solidify (at a higher temperature) have a different composition than the last crystals to solidify (at a lower temperature). In industry, this principal is used to separate out impurities and create very pure substances.

In any case, as feldspars cool and form crystals, the composition of the crystals being formed changes. In the real world, the temperature of the forming crystals can fluctuate up and down. This helps create layers inside the crystal with slightly different compositions.

Labradorite forms when the crystals contain a mixture of sodium and calcium. As labradorite forms into a crystal, the composition of calcium and sodium varies. The changes in composition forms layers inside the labradorite crystals. The layers inside the crystal affect light as it travels through the crystal. If you look at the layer from the right angle, you see the Schiller effect – the flash of color.