polarizing jewelry

Remember "I want my MTV"?  Not missing that much anymore, but since moving to West Virginia, I am missing my JTV. 

JTV - Jewelry television - is a 24-hour shopping network devoted to jewelry and gemstones.  I miss it in a sort of a "so bad it's good" way.  Some of the channel hosts are truly remarkable for the interesting information they provide.  For instance, the woman who spent twenty minutes talking about this 'gemstone' (actually a piece of shell) that was found in only ONE PLACE ON EARTH.  That's right - this amazing piece of bright red all-natural (except for having been dyed) material was only found in one single isolated place on God's Green Earth:  The Pacific Ocean.  (The Pacific Ocean covers about 30% of the Earth's surface.)  The same woman also suggested that rutilated quartz - titanium oxide slivers inside quartz (silicon oxide) get there the same way that a straw can penetrate a tree during a tornado.

I've actually never bought anything from JTV, but it certainly has fueled my interest in shiny colored rocks. Yeah, diamond has a very high index of refraction that makes it sparkle like crazy, but there are a lot more interesting (and less expensive) minerals that play much better tricks on the light entering them.

Shine light through a piece of green tourmaline.  To absolutely no one's surprise, a faint greenish light emerges from the other side.  Now place a second piece of tourmaline after the first one.  The amount of light that gets through the second mineral depends on the relative orientations of the two pieces of stone.  When the two pieces are approximately 90° with respect to each other, no light passes through.  When the two pieces are in the same orientation, a lot of light passes through them.  Isaac Newton suggested that this behavior had something to do with ‘sides’ or ‘poles’ of the light and called the phenomenon ‘polarization’.

Light is made up of electric and magnetic fields that orient themselves in a mutual three-way perpendicular set-up with the direction of propagation.  If the light wave is moving away from you in the diagram at left, then the electric field and magnetic field will oscillate in the plane of the screen, always maintaining right angles with each other.

They can, however, take on an infinite number of directions while still maintaining the required perpendicularity, as the pictures in the middle and at the bottom show.  Your typical light beam contains many different light waves and each wave can point in any direction.  This is called unpolarized light because all directions are present.  Polarizing the light is limiting the allowed directions. 

A beam of unpolarized sunlight can be so bright that it's blinding.  Polarized sunglasses block the light waves that aren't oriented in the direction selected by the lens.  You're sampling a fraction of the light, so you get all the information, but without all the intensity.

You can imagine a mineral like tourmaline (a boron silicate with small impurities that change the color) as acting like a very small slit:  only one orientation of electric field (one polarization) is allowed to pass.  If a slit is parallel to the polarization of the wave, the wave gets through, but the slit blocks waves whose electric fields aren't oriented in the correct direction. 

In the diagram below, the two pieces of tourmaline on the left are oriented so that they let the same polarization of light through.  On the right, the first piece lets through one polarization of light, but the second piece blocks that polarization and nothing gets through.  Here's a neat applet that lets you investigate this phenomenon more and shows what a bad artist I am.

Tourmaline does make pretty jewelry, but showing off this effect is not so easy.  First, you need two pieces of nicely crystalline tourmaline and second, you have to worry about how they are oriented.  Third, you can show the same phenomenon even better with two polarized sunglasses lenses.  Maybe not so impressive.

Light entering a material bends when it travels from air into the materials.  The classic illustration of refraction is the pencil in the half-full glass of water:  the pencil looks like it's broken because the light waves that travel through the water bend differently than the light waves that travel through the air. 

When light enters a tourmaline, for example, the wavelengths of light that correspond to green are reflected, which is why you see the tourmaline as green.  The other wavelengths of light are absorbed.  When light reflects from a very thin film of a liquid like oil, some of the light reflects from the surface of the oil and some travels through the oil and reflects off the interface between the oil and, say, water.  The oil film makes the two rays of light travel different distances, which causes some colors to appear stronger and other colors to disappear. 

I took the photo the oil film at right during a holiday break visit to the USS Arizona Memorial.  Almost 70 years after having been sunk, the ship leaks about a quart of oil every day out of an estimated half-million gallons still within the ship.

I covered the case of thin-film reflection in some detail in The Physics of NASCAR - (you can see the relevant page here).  The application was paint that changes colors depending on the direction you view it from. In this case the critical parameter is the thickness of the film relative to the wavelength of light.  Different wavelength means different color.

When you deal with light in gemstones (as opposed to thin films), you have to worry about the internal structure of the stone.  One of the most beautiful things about gemstones is that (prior to faceting and polishing), they are excellent examples of crystal structure.  A crystal is a regularly arranged collection of atoms that repeats over and over. 

A simple material, like salt, has a base structure made of cubes.  The picture at left shows the beautiful cubic nature of NaCl, complete with sharp edges.  You can see similar symmetric beauty in minerals like galena.  Those sharp edges are a direct result of the atomic-level cubic order.

Not all materials have nice simple crystal structures, but materials do have a limited number of general forms - sort of a Platonic series of crystal structure.  When we draw crystal structures, we usually omit the actual atoms, so you have to imagine an atom or a collection of atoms at each vertex in the drawings below.

The simple cubic structure is shown at left.  The lengths of each side are the same and the angles between the sides are 90 degrees.  It doesn't really matter which side you look at it from because all the sides are the same.  The shape is highly symmetric.  I can rotate it 90 degrees about any axis and get the same shape back.  It has reflection symmetry: if you draw a vertical line through its center, the left half is an exact reflection of the right half.

The middle shape, which is called tetragonal, is obtained by stretching a cube along one direction.  We've broken symmetry because we can tell the difference between the ends and the side now.  Finally, the triclinic shape (on the right) is just plain messed up.  All three axes are different lengths and there are no 90-degree angles.  There's very little symmetry.  Or at least, that's how I tried to draw it.

A  lack of crystal symmetry can have some fascinating effects.  Different directions of the crystal treat light differently.  Tetragonal (and hexagonal) minerals can be dichroic, meaning that they show two different colors depending on which direction you view the mineral from.   (Hexagonal is like a cylinder, but with six sides: It's what you would get if you extruded pasta through a hexagonal die.)

Iolite ((Mg,Fe)₂Al₄Si₅O₁₈) is an orthorhombic structure, which means that its three axes make 90-degree angles with each other; however, the lengths of the three sides are each different.  Iolite has extremely strong strong trichroism:  It looks bluish-purple if you view it from one direction, yellow when viewed from another, and almost crystal clear in the third direction.  Only minerals that have triple asymmetry can be trichroic.  The effect is strongest in the highest quality single crystals. 

Usually, jewel cutters select the direction that gives you the best looking color and cut the stone so that it will display that color.  That's why you normally see Iolite jewelry being a rich, deep purple-blue.  Pleochromisn can make cutting the stone a challenge.

So instead of trying to beat nature, why not work with it?  Andalusite (Al₂SiO₅) is also orthorhombic, and looks yellow, olive green, or red depending upon which axis you're looking along.  The individual different colors aren't all that spectacular: they're not the vibrant, rich colors you see in traditional gemstones.  But: if you facet the stone just right, you can get different facets to reveal different colors simultaneously.  The colors change when the stone moves.  It's an unusual effect much appreciated by those of us bored with standard bling.

Another positive:  the stone at left (which is for sale) is about 2 carats and costs about $200.  You get a lot more interesting behavior for the dollar than a boring ol' diamond.  (Keep that in mind as Valentine's Day approaches.)

You can tell if a mineral is pleochroic by looking at it with a polarizing microscope.  A polarizing microscope passes the light beam incident on the sample through a polarizer, so instead of looking at your sample with unpolarized light, you're looking at it with polarized light.  In a piece of rough that has multiple crystals (meaning the crystal axes are oriented in different directions), the colors changes as you rotate the direction of polarization.

The polarization effect is responsible for Iolite being nicknamed 'Viking's Compass'.  Imagine you are trying to navigate in the far North oceans as winter is approaching and the sky appears to be twilight 24 hours a day. The Sun has dipped down below the horizon, but is still bright enough that you can't see the stars.  Add a persistent fog.  How is a Viking to locate the next place to pillage?

Legend has it that Iolite crystals were used as polarizing filters to allow sailors to determine the precise position of the Sun when the Sun wasn't visible.   Iolite, remember, appears bluish-purple, yellow and almost clear along its three directions.  Natural Iolite crystals are smaller than an average pinky finger, but long and pointy like quartz crystals.  If the polarization of light is along the long axis of an Iolite crystal, blue light is absorbed more strongly than yellow light, so the stone looks yellow.  Light scattered by air molecules becomes polarized in a direction that is at right angles to a line that stretches from you to the Sun.

When the stone looks blue, it means that the long axis of the crystal is pointing toward the Sun.  There are two ends to the crystal, so that narrows the places the Sun could be down to two.  If you're so confused that you can't intuit which of two directions that differ by 180 degrees is the direction the Sun is in, you shouldn't be a Viking commander anyway.

Phenomena like this make it easier to understand why people ascribe mystical powers to gems.  When the Iolite crystal is pointing in the direction of the Sun, it 'turns' blue, even on a cloudy day.  It would take centuries before we even know light was a wave, much less a polarized wave.  That didn't keep the Vikings from figuring out phenomenologically that you could use the stone as a tool.

I love having interesting jewelry (or stuffed bacteria on my desk) to stimulate a conversation about science.  Unfortunately, Iolite is one of those stones that is almost always cut to display the purple-blue color, so I'm having difficulty finding a natural spear of Iolite that I can make into a pendant. 

Just in case I'm ever navigating the North Pole as winter approaches and my GPS is broken.

Comments

[Barry]

Terrific and enlightening article as always, Di. How about covering circular polarization? It's the primary technique used for 3D movies in the theaters and could use some thoughtful explanation.

[William Tinsley]

Gems and minerals are amazing and can create so much pleasure. I really enjoyed your article and hope that more people get involved with gemstones.
I am not an "expert" in any sense of the word, but I did enough jewelry as a hobby that I decided to share some pieces with everyone that may be interested. My new website is at http://www.rockhoundgemstones.com A lot of my pieces are made with beaded gemstones, but many are made from the rough. We go rockhounding to gather the rocks/gems that we use in our jewelry. It is so interesting and it is just unbeliveable how many beautiful stones that Mother Nature has given us to enjoy. We are in our 60's and are still very active in rock hunting and making jewelry. I hope some of you will go to my site and browse through the products and leave a message on my blog page. I am still learning more everyday, so leave suggestions and ideas to help. Hope to hear from you soon!

[Tuomas]

Great article, and I have enjoyed your posts a lot in the past.

However, in this post there's one glaring factual mistake: "A beam of unpolarized sunlight can be so bright that it's blinding. Polarized sunglasses block the light waves that aren't oriented in the direction selected by the lens. You're sampling a fraction of the light, so you get all the information, but without all the intensity."

This would not help glare so much alone. The reason for polarizing sunglasses is that most of the glare from, e.g., a shiny road surface, is polarized horizontally. For example, circularly polarizing sunglasses would *not* help at all.

See http://www.polarization.com/water/water.html

[AD]

Erm...I know you're talking about Iolite, but being a denizen of the internet as I am, I couldn't help reading it as Lolite at first glance. That would be a pretty awesome mineral.

[[email protected]]

LOLite! That sounds like something we need to invent! But what would be in it? Lithium - Iron? Lanthanum-Silver-Fluorine?

[David Wiley]

Thank you ever so much for introducing me to possibly the most dangerous website ever! :-) My wallet hates you, but my wife will never be short of pressies again.

--
The thing that still freaks me out about polarization is take the example above with two polarizers set at 90 degrees to each other: no light passes. Now put another polarizer in between at a 45 degree angle to both. Light once again can pass. Quantum mechanics is a bitch! The 60 Symbols video series has a good video on plane polarization: http://bit.ly/hE6oaG