Topic
Optics
Optics entries on Zultpedia document the optical phenomena of color-change diaspore: the alexandrite effect, refractive index 1.682–1.752, strong birefringence ~0.048, dispersion, biaxial-positive optic character, and the wavelength-selective absorption produced by trace iron and chromium chromophores.
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Education Article
How to Photograph Zultanite Color Change
Photographing Zultanite color change requires three illuminant setups: daylight (D65, 6500 K), indoor incandescent (2856 K), and candlelight (~1850 K). Use a neutral-gray background, manual white balance, fixed exposure, and the same…
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Gem Article
How Master Cutters Orient Diaspore Rough
Master cutters orient color-change diaspore rough so the table facet is perpendicular to the direction of strongest color change while keeping the {010} cleavage plane parallel to the girdle (not the table).…
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Glossary Term
Chromophore
The atomic or molecular structural feature responsible for a material's color. In gem-quality color-change diaspore, the chromophores are trace iron (Fe³⁺) and chromium (Cr³⁺) substituting into the aluminum site of the diaspore…
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Glossary Term
Asterism
The optical phenomenon of a star-shaped reflection appearing on the surface of a cabochon-cut gemstone, caused by light scattering from oriented inclusions. Asterism is not characteristic of color-change diaspore; the gem is…
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Glossary Term
Dispersion
The separation of white light into spectral colors as it passes through a transparent material, producing the "fire" visible in cut gemstones. Diaspore has moderate dispersion (~0.020), lower than diamond (~0.044) but…
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Glossary Term
Optic Axis
A direction within an anisotropic crystal along which light travels without splitting into two rays. Biaxial gems like diaspore have two optic axes. The cutter orients the gem so the optic axes…
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Glossary Term
Lustre
The character of light reflected from a gemstone's polished surface, ranging from dull through vitreous (glass-like) to adamantine (diamond-like). Diaspore shows a vitreous to subadamantine lustre when polished, related to its refractive…
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Glossary Term
Color Change
Color change in Zultanite — also called the alexandrite effect — is the appearance of distinctly different colors in the same gem under different illuminants: sage-green under cool daylight (6500 K), champagne-gold…