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When I am searching the colour of $\ce{Mg3N2}$ I came across photos showing different colours: greenish-yellow on Wikimedia, and some other just yellow on some Google Images results.

I am eager to know what causes some samples to be bit greenish.

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    $\begingroup$ Maybe due to impurities, the color differs. $\endgroup$ – Nilay Ghosh Dec 26 '19 at 4:37
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    $\begingroup$ Particle size and polymorphism would also affect the color of a solid. $\endgroup$ – andselisk Dec 26 '19 at 5:55
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    $\begingroup$ Privacy note: be very cautious when you post a link to the Google search results. Not only the address string looks messy, it also contains private info such as your device and browser names in plane text. There are several browser extensions that fix that and create a short nice-looking URL without using services like tinyurl; at the moment, I personally use ClearURLs. $\endgroup$ – andselisk Dec 26 '19 at 6:12
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Minor differences in color are likely either due to impurities or due to intrinsic crystal defects such as f-centers. For example, $\ce{ZnO}$ is often a shade of yellow, rather than pure white, due to those defects. In alkali halides, h-centers are similar defects giving rise to colors. You can easily create color centers in $\ce{NaCl}$ with a spark coil in just a few minutes.

Another possibility is that there are mixtures of different allotropes, or polymorphic forms. An extreme example is transparent diamond vs. black graphite. BTW, diamonds having varied colors due to impurities (e.g. nitrogen) and to intrinsic defects are useful in studying quantum phenomena, as well as gems.

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