How Do These Colors Happen?

[iLander]

So I was looking through Art Cut Gem's new stones (http://www.artcutgems.com/category.php?new=1&page=2) particularly the bi-colored tourmalines on page 2. This question kept occurring to me: How?

What exactly is going on inside the crystal to make it switch colors right in the middle of it's growth? Does it stop and start growing again? Or does it cool off then heat up again? Is there a puddle of various chemicals and half the crystal is poking out of the vat? Did nature somehow dip it into something (I know that doesn't even sound plausible, but I'm out of ideas!)?

I can't figure this out and it's making me crazy.

How rare are these bicolors, anyway? Why are so many of them red and green?

 

[chrono]

I have read that mineralogists have been unable to determine exactly what force of nature causes the dramatic colour change during the crystallization process. Pink + green seems to be the most common, with a rare blue + green and even a tri-colour which shows gray + green + blue. In general, tourmaline pegmatite is formed as coarse grained granite by slow cooling of magma deep within the earth. Large crystals are formed from the original molten residue as the rocks cool at different rates within the crust. As the temperature drops, the minerals are separated out and tourmaline crystals will begin to form. Some are found as alluvial deposits after being washed down by rain, wind and water/river.

 

[Michael_E]

Tourmaline crystals grow in cavities which are filled with hot water that contains a wide range of different elements. They grow from the wall of the cavity inwards as well as growing in width. If the temperature or composition of elements in the cavity changes, then the crystal changes as well, (sometime in color, sometimes even in shape. Color changes occur with differing elements in the water and can occur over in the length of the crystal, looking like multi-colored sticks, or over the width of the crystal, looking like a slice of watermelon in cross section. There are many variations on this theme with phantoms, included crystals of other species and even strange shape changes. Very interesting isn't it?

 

[Treenbean]

This is a cool thread. I wondered this myself and thought it would have to do with the environment of the crystal during it's growth. I found a cool paper too that helps explain the colors of gems and even the chemical composition and bonds in the crystal structure.
http://scifun.chem.wisc.edu/chemweek/PDF/Gemstones.pdf

 

[Richard M.]

The coarse-grained granites Chrono refers to are called pegmatites. They’re created when hot volcanic magma penetrates into cracks in the earth but don’t make it to the surface. So they sit, trapped, for many thousands of years and slowly cool. That’s a perfect environment for the growth of some gem crystals like tourmaline. The rule is: fast cooling = small crytals; slow cooling = large crystals.

As the crystals slowly grow the chemicals in the solutions that nourish them often change. As one becomes depleted the crystal uses whatever other chemical(s) may be available, accounting for the color changes. Some multicolored tourmaline crystals have been reported with as many as 6 different hue changes along their length.

What specifically causes pink and green colors? That’s a complex question. There are at least 7 causes of pink, while iron is usually given as the color-causing agent in green tourmaline. But there are many variables.

Sourcing good bicolored and multicolored tourmalines is more difficult from year to year and prices are going up as demand increases, at least in my experience. Mining in many areas that used to produce fine tourmaline, especially Brazil, has been severely restricted for environmental and other reasons.

 

[minousbijoux]

I love this forum! Ilander, thanks so much for asking the question - I love how much there is to learn and that you experts take the time to teach. Treenbean, it is WAY COOL!

 

[iLander]

(Treenbean, minousbijoux: I know, isn't it great, you can just ask a question and have it answered by smart people? How cool is that?)

Back to the experts:

So, in other words, nature really does have a vat of chemicals?

That's pretty bizarre . . . and kind of awesome . . .

So what are we talking about, in terms of time? Millions of years, thousands? How old are these and all those other gems out there?

 

[Richard M.]

The time required for crystal formation in pegmatites depends on many factors. Since no one has actually observed the process it’s mostly guesswork. It’s known that certain crystals like quartz can be lab-grown in hot water solutions (hydrothermal) in a matter of months. The time may vary a lot for natural crystal formation.

Keep in mind that while gem minerals may form rather quickly, it may take millions of years for the surrounding rock to erode enough to expose the formations.

This movie illustrates the process: http://nature.berkeley.edu/classes/eps2//wisc/movie/pegmatites.mov

For more information on other methods of gem crystal formation, go here: http://nature.berkeley.edu/classes/eps2//wisc/Lect3.html

The other movies are very helpful in understanding these processes in general.

Richard M. (Rick Martin)

 

[JewelFreak]

That is totally fascinating. Thanks for asking, iLander, and for answers & references, expert people. Very cool to get smart answers to smart questions.

--- Laurie