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Artificially grown bismuth crystals are arguably magnificent, as seen in the image below:

enter image description here

Image source: Minerals.net

The website states that:

They have hopper-like growths in pseudocubic crystals and are usually coated with chemicals to prevent tarnish, thus maintaining the silver-white color. Sometimes the coating gives a colorful effect on the bismuth.

What coating gives the colourful iridescence? and what chemical processes cause the colourful iridescence to occur?

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The colour is from a thin film of bismuth(III) oxide that forms on the surface if the crystals are formed in air. At the elevated temperatures used to melt bismuth, the oxide forms quite quickly. The iridescence is a result of thin film interference—light waves constructively or destructively interfere as they bounce off the bismuth-oxide and oxide-air interfaces. The interference is dependant on the wavelength and incidence angle of light as well as the film thickness, which results in different wavelengths of light being attenuated by different amounts, making the surface appear coloured and the colour changing with viewing angle.

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    $\begingroup$ Though not nearly as stunning, regular aluminium bars can sometimes also show the same effect with the correct illumination and viewing angles. However, thin film interference can be greatly increased in many metals by electrochemically forced oxidation, forming a layer of oxide with precisely controlled thickness. This process is called anodizing or anodization, and can be used to colour metals without using dyes. A great video explaining this process, by Bill Hammack the Engineer Guy, can be found here. $\endgroup$ – Nicolau Saker Neto Jun 5 '15 at 16:24
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    $\begingroup$ Yes, there are a few metals that make very nice colours from interference effects with oxide coatings, especially titanium and niobium. $\endgroup$ – Michael DM Dryden Jun 5 '15 at 17:31

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