<p>I'd like to elaborate of the <strong>Chemicals</strong> issue of Azzie Rogers' answer.</p>

<p>You can divide the chemical coloring into three main parts <em>(there may be more, but these are the important ones)</em>:</p>

<p><strong>Inclusions</strong></p>

<p>A large, solid crystal can have tiny inclusions of other solid minerals. Commonly these inclusions are too small to individually observe by the naked eye. Microscopic methods, either optic or electronic, are usually required to properly identify the inclusions. Nonetheless, when a crystal is viewed on a macroscopic scale, the inclusions give their characteristic color to the mineral.</p>

<p>Here is one great example:</p>

<p><img src="https://i.sstatic.net/H0178.jpg" alt="enter image description here"></p>

<p><sup>(<a href="http://dx.doi.org/10.1017/S0016756813000800" rel="noreferrer">Fig. 4 of Anenburg <em>et al.</em> 2014</a>)</sup></p>

<p>You can see the blue celestine (strontium sulfate) in the bottom, and the white calcite (calcium carbonate) on top. Their interface is colored red and salmon pink. The reason is the existence of tiny micrometer sized inclusions of iron oxides (<em>i.e.</em> rust). Note that the iron oxides are different minerals, <em>e.g.</em> hematite, goethite, etc.</p>

<p>Mineral inclusions can sometimes cause even more spectacular effects, such as asterism:
<img src="https://i.sstatic.net/fb9Jv.jpg" alt="enter image description here">
<sup> (<a href="https://www.flickr.com/photos/charlestilford/2328058336/" rel="noreferrer">source: Charles Tilford</a>)</sup></p>

<p>In this case, the cause are tiny inclusions of rutile (titanium oxide).</p>

<p><strong>Crystal field theory</strong> (allochromatism)</p>

<p>The crystal field theory explains, among other things, what happens to light as it encounters metals in different structural configurations inside a crystal. Keeping the complicated terminology at minimum (<em>energy level splitting yay!</em>), what basically happens is that different metal cations (positively charged atoms) in a crystalline structure absorb different wavelengths of light. The identity of the metal is not the only important thing: a paramount factor is the number of oxygen atoms that surround the metal atoms and their shape (be it a tetrahedron, octahedron, etc.) These metals are not part of of the formal chemical composition of the mineral. Instead, they replace other atoms in the mineral by very little amounts. For example, ruby is a mineral composed of aluminum oxide. If you take just a bit of this aluminum and replace it with chromium (less than 1% is enough), it becomes vivid red. Note that in contrast to inclusions, we are talking about different metals <em>in the same mineral</em>, and not inclusions of <em>different minerals</em>.</p>

<p>A digestible explanation with some nice photographs and applets can be found here: <a href="http://www.chm.davidson.edu/vce/coordchem/cft.html" rel="noreferrer">http://www.chm.davidson.edu/vce/coordchem/cft.html</a></p>

<p><strong>Idiochromatism</strong></p>

<p>This is the case where the color of the mineral is not dictated by trace amounts of metals in an otherwise colorless mineral (see the ruby example above), but rather by the major chemical constituents of the mineral. A great example is copper, which gives strong green colors to some minerals that contain it (<em>e.g.</em> malachite):</p>

<p><img src="https://i.sstatic.net/4V4z9.jpg" alt="Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons"></p>

<p><sup>(<a href="http://commons.wikimedia.org/wiki/File%3AMalachite-41365.jpg" rel="noreferrer">Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons</a>)</sup></p>

<p>Or the red color of cinnabar, given by mercury:</p>

<p><img src="https://i.sstatic.net/EYEkC.jpg" alt="Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons"></p>

<p><sup>(<a href="http://upload.wikimedia.org/wikipedia/commons/4/4f/Cinnabar-157604.jpg" rel="noreferrer">Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons</a>)</sup></p>

CC BY-SA 3.0
2014-11-20T16:29:20.370Z
That beautiful! You answered it beautifully and adding in the pictures made it really great! :)
L.B.
CC BY-SA 3.0
2014-11-08T11:39:36.233Z
2014-11-08T11:39:36.233Z
Gimelist
<p>The reason minerals like quarts and diamonds vary in color is generally caused by the chemical elements involved while the crystal is being formed.</p>

<p><strong>Chemicals</strong><br>
Different colors can be created by different chemicals. Amethyst for example has traces of iron built into its crystalline structure giving it a purple hue. Iron can also give crystals a yellow hue.</p>

<p><em>[edit: See Gimelist's answer for a much more detailed and colorful explanation]</em></p>

<hr>

<p><strong>Growth Imperfections</strong><br>
Some colors, like in smoky quartz, are from growth imperfections. These imperfections change the way the crystal reflects light, which changes the appearance of coloration.</p>

<hr>

<p><strong>Light</strong><br>
Part of the color seen when looking at a crystal is light. When light enters a crystal its spectrum is broken apart, and part of it is absorbed while other parts are reflected. This changes the <em>apparent</em> color of the crystal.</p>

CC BY-SA 3.0
2014-04-19T08:49:17.460Z
I'm not a chemist by any means, but isn't it the case that the same element or compounds can exhibit different colours when in different chemical configurations? Also, I wonder if this question isn't better suited to chemistry.SE?
naught101
CC BY-SA 3.0
2014-04-29T21:15:25.603Z
With respect to variations in colour with chemical origin, it is perhaps worth noting that it is almost always [transition metals](https://en.wikipedia.org/wiki/Transition_metal) that are responsible for these variations.
kaberett
CC BY-SA 3.0
2014-11-08T10:45:10.943Z
"Amethyst for example has iron deposits built into its crystalline structure giving it a purple hue." I think that "deposits" is not the correct term here. "Crystals that have sulfur built into their crystalline structure have a yellow hue." Absolutely not! Some may be yellow, but it's not related to the sulfur.
Gimelist
CC BY-SA 3.0
2014-11-20T14:48:44.153Z
@Michael thanks for that comment. (I didn't see it before) I checked again, and I can't seem to find the source that said it was sulfur that made it yellow. I thought it was a reliable source... Anyway, thanks for that tip. :)
Azzie Rogers
CC BY-SA 4.0
2014-04-18T22:09:49.647Z
2019-01-22T04:05:35.457Z
2019-01-22T04:05:35.457Z
Azzie Rogers
<p>Some crystals, like quartz and diamonds, form in a variety of different colors. For example, quartz can be clear, 'foggy', pink, yellow, and even blue.</p>

<ul>
<li>What happens during the crystal's formation to make it one color or
another?</li>
</ul>

CC BY-SA 3.0
2014-04-23T01:40:20.987Z
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
2014-04-23T13:32:18.950Z
@naught101 I have seen the question, but I agree with the answer personally.
Azzie Rogers
CC BY-SA 3.0
2014-04-23T21:38:41.883Z
Yeah, I do too now :)
naught101
<geology><crystals><crystallography><diamond>
Why do crystals, like quartz and diamonds, form in different colors?
Azzie Rogers
# Why do crystals, like quartz and diamonds, form in different colors?

(Asked by: username_0 on 2014-04-18)

Some crystals, like quartz and diamonds, form in a variety of different colors. For example, quartz can be clear, 'foggy', pink, yellow, and even blue.

* What happens during the crystal's formation to make it one color or
  another?

**Tags:** geology, crystals, crystallography, diamond

> **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_0**: @username_1 I have seen the question, but I agree with the answer personally.  
> **username_1**: Yeah, I do too now :)  

---

## 2 answers

### Answer by username_0 on 2014-04-18. Score: 12 | Accepted answer

The reason minerals like quarts and diamonds vary in color is generally caused by the chemical elements involved while the crystal is being formed.

**Chemicals**  
Different colors can be created by different chemicals. Amethyst for example has traces of iron built into its crystalline structure giving it a purple hue. Iron can also give crystals a yellow hue.

*[edit: See username_2's answer for a much more detailed and colorful explanation]*

---

**Growth Imperfections**  
Some colors, like in smoky quartz, are from growth imperfections. These imperfections change the way the crystal reflects light, which changes the appearance of coloration.

---

**Light**  
Part of the color seen when looking at a crystal is light. When light enters a crystal its spectrum is broken apart, and part of it is absorbed while other parts are reflected. This changes the *apparent* color of the crystal.

> **Comments:**
> **username_1**: I'm not a chemist by any means, but isn't it the case that the same element or compounds can exhibit different colours when in different chemical configurations? Also, I wonder if this question isn't better suited to chemistry.SE?  
> **username_4**: With respect to variations in colour with chemical origin, it is perhaps worth noting that it is almost always [transition metals](https://en.wikipedia.org/wiki/Transition_metal) that are responsible for these variations.  
> **username_2**: "Amethyst for example has iron deposits built into its crystalline structure giving it a purple hue." I think that "deposits" is not the correct term here. "Crystals that have sulfur built into their crystalline structure have a yellow hue." Absolutely not! Some may be yellow, but it's not related to the sulfur.  
> **username_0**: @Michael thanks for that comment. (I didn't see it before) I checked again, and I can't seem to find the source that said it was sulfur that made it yellow. I thought it was a reliable source... Anyway, thanks for that tip. :)  

---

### Answer by username_2 on 2014-11-08. Score: 9

I'd like to elaborate of the **Chemicals** issue of username_0 username_0' answer.

You can divide the chemical coloring into three main parts *(there may be more, but these are the important ones)*:

**Inclusions**

A large, solid crystal can have tiny inclusions of other solid minerals. Commonly these inclusions are too small to individually observe by the naked eye. Microscopic methods, either optic or electronic, are usually required to properly identify the inclusions. Nonetheless, when a crystal is viewed on a macroscopic scale, the inclusions give their characteristic color to the mineral.

Here is one great example:

![enter image description here](https://i.sstatic.net/H0178.jpg)

([Fig. 4 of Anenburg *et al.* 2014](http://dx.doi.org/10.1017/S0016756813000800))

You can see the blue celestine (strontium sulfate) in the bottom, and the white calcite (calcium carbonate) on top. Their interface is colored red and salmon pink. The reason is the existence of tiny micrometer sized inclusions of iron oxides (*i.e.* rust). Note that the iron oxides are different minerals, *e.g.* hematite, goethite, etc.

Mineral inclusions can sometimes cause even more spectacular effects, such as asterism:
![enter image description here](https://i.sstatic.net/fb9Jv.jpg)
 ([source: Charles Tilford](https://www.flickr.com/photos/charlestilford/2328058336/))

In this case, the cause are tiny inclusions of rutile (titanium oxide).

**Crystal field theory** (allochromatism)

The crystal field theory explains, among other things, what happens to light as it encounters metals in different structural configurations inside a crystal. Keeping the complicated terminology at minimum (*energy level splitting yay!*), what basically happens is that different metal cations (positively charged atoms) in a crystalline structure absorb different wavelengths of light. The identity of the metal is not the only important thing: a paramount factor is the number of oxygen atoms that surround the metal atoms and their shape (be it a tetrahedron, octahedron, etc.) These metals are not part of of the formal chemical composition of the mineral. Instead, they replace other atoms in the mineral by very little amounts. For example, ruby is a mineral composed of aluminum oxide. If you take just a bit of this aluminum and replace it with chromium (less than 1% is enough), it becomes vivid red. Note that in contrast to inclusions, we are talking about different metals *in the same mineral*, and not inclusions of *different minerals*.

A digestible explanation with some nice photographs and applets can be found here: <http://www.chm.davidson.edu/vce/coordchem/cft.html>

**Idiochromatism**

This is the case where the color of the mineral is not dictated by trace amounts of metals in an otherwise colorless mineral (see the ruby example above), but rather by the major chemical constituents of the mineral. A great example is copper, which gives strong green colors to some minerals that contain it (*e.g.* malachite):

![Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons](https://i.sstatic.net/4V4z9.jpg)

([Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons](http://commons.wikimedia.org/wiki/File%3AMalachite-41365.jpg))

Or the red color of cinnabar, given by mercury:

![Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons](https://i.sstatic.net/EYEkC.jpg)

([Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons](http://upload.wikimedia.org/wikipedia/commons/4/4f/Cinnabar-157604.jpg))

> **Comments:**
> **username_3**: That beautiful! You answered it beautifully and adding in the pictures made it really great! :)  

---

