<p>The color of a mineral can be caused by a variety of mechanisms. This is also true of amethyst, which is a variety of quartz ($\ce{SiO2}$), and can be found in many colors. </p>

<p>The major factors responsible for the <a href="http://www.enmu.edu/services/museums/miles-mineral/colors.shtml" rel="nofollow">production of color</a> in minerals fall into five categories:</p>

<ol>
<li>The presence of an element essential to the mineral composition</li>
<li>The presence of a minor chemical impurity</li>
<li>Physical defects in the crystal structure</li>
<li>The mechanical mixture of very fine impurities</li>
<li>The presence of finely-spaced structures in the mineral</li>
</ol>

<p>See these pages for more information about the <a href="http://minerals.gps.caltech.edu/COLOR_Causes/index.html" rel="nofollow">causes of color in minerals</a>.</p>

<p>If you search for "Color Centers in Quartz" you will find a large literature on this subject. Some color centers in quartz can be created or destroyed by irradiating crystals with light or X-rays. </p>

<p>The quantitative measurement and study of mineral color is called <a href="http://minerals.gps.caltech.edu/" rel="nofollow">Mineral Spectroscopy</a>.   </p>

CC BY-SA 3.0
2014-11-08T11:50:15.117Z
How does this answer the question?
Gimelist
CC BY-SA 3.0
2014-04-22T17:12:21.103Z
2014-12-19T10:02:13.460Z
2014-12-19T10:02:13.460Z
Mark Rovetta
<p>It is well known that the purple hue that is characteristic of the amethyst quartz ($\ce{SiO2}$) will fade if the crystal is exposed to light for a prolonged period of time.  What is the underlying mechanism of this change?</p>

CC BY-SA 3.0
2014-04-23T01:39:45.943Z
I feel like this question is better suited to Chemistry.SE. See discussion at http://meta.earthscience.stackexchange.com/questions/164/when-are-questions-about-crystals-on-topic
naught101
CC BY-SA 3.0
2014-12-19T12:18:28.670Z
"It is well known"... to most people, maybe.  I just learned something new! :)
Richard
CC BY-SA 3.0
2014-12-23T15:23:31.217Z
@naught101 I agree. Though this phenomenon does occur naturally, it is a set of chemical reactions that is mostly responsible for this.
unbindall
<mineralogy><minerals><crystals><crystallography>
Why does the colour of amethyst fade if exposed to too much light?
Kenshin
# Why does the colour of amethyst fade if exposed to too much light?

(Asked by: username_0 on 2014-04-19)

It is well known that the purple hue that is characteristic of the amethyst quartz ($\ce{SiO2}$) will fade if the crystal is exposed to light for a prolonged period of time. What is the underlying mechanism of this change?

**Tags:** mineralogy, minerals, crystals, crystallography

> **Comments:**
> **username_1**: I feel like this question is better suited to Chemistry.SE. See discussion at http://meta.earthscience.stackexchange.com/questions/164/when-are-questions-about-crystals-on-topic  
> **username_2**: "It is well known"... to most people, maybe.  I just learned something new! :)  
> **username_3**: @username_1 I agree. Though this phenomenon does occur naturally, it is a set of chemical reactions that is mostly responsible for this.  

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## 1 answers

### Answer by username_4 on 2014-04-22. Score: 4

The color of a mineral can be caused by a variety of mechanisms. This is also true of amethyst, which is a variety of quartz ($\ce{SiO2}$), and can be found in many colors.

The major factors responsible for the [production of color](http://www.enmu.edu/services/museums/miles-mineral/colors.shtml) in minerals fall into five categories:

1. The presence of an element essential to the mineral composition
2. The presence of a minor chemical impurity
3. Physical defects in the crystal structure
4. The mechanical mixture of very fine impurities
5. The presence of finely-spaced structures in the mineral

See these pages for more information about the [causes of color in minerals](http://minerals.gps.caltech.edu/COLOR_Causes/index.html).

If you search for "Color Centers in Quartz" you will find a large literature on this subject. Some color centers in quartz can be created or destroyed by irradiating crystals with light or X-rays.

The quantitative measurement and study of mineral color is called [Mineral Spectroscopy](http://minerals.gps.caltech.edu/).

> **Comments:**
> **username_5**: How does this answer the question?  

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