Colour-change imitation Zultanite

Zultanite is the commercial name for a variety of diaspore (a hydrated aluminium oxide) coloured by manganese. The gem material is a recent discovery originating from a single mine in central Turkey and, because of its rarity, is expensive on the open market. Its most endearing characteristic is a remarkable colour change from honey-pink to green depending on the spectral properties of the illuminating light source.

While colour-change gems are difficult to imitate, a reasonable but rather "too-good-to-be-true" imitation for Zultanite can be fashioned from a type of neodymium-doped glass used to construct lasers.

The image above shows two photographs of a cut imitation Zultanite compared with a piece of the lanthanide-doped glass. The left image is illuminated with a tungsten (W) filament lamp while the right is taken under an ordinary energy-saving mercury (Hg) bulb. The white balance for the left image is 'tungsten' and the right hand image is matched to it.

The thin-grey spectrum above shows the optical transmission of the 'Zultanite' (which is essentially identical to that of the laser-glass) while the thicker brown and green spectra show the flux spectra of the tungsten and mercury lamps transmitted by the gem. The green colour originates predominantly from the high tranmittance at the wavelength of the 546nm Hg emission line in the mercury lamp.

This is one of the most remarkable cases of colour change seen in a material illuminated by lamps which, to the human eye, appear almost identical.

The only significant difference I can see between the cut imitation and the laser glass sample is the strength of the fluorescence under 404nm laser excitation. The 'Zultanite' is almost inert while the laser glass fluoresces red.

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