Topic
Mineralogy
Mineralogy entries on Zultpedia document the species-level identity of color-change diaspore: chemistry (α-AlO(OH)), crystal system (orthorhombic), refractive indices, pleochroism, perfect cleavage on {010}, and the trace iron + chromium chromophore mechanism that produces the alexandrite effect. Sourced from primary mineralogical literature (Mindat, Mineralogical Magazine, Wikipedia’s peer-reviewed mineral pages) and the Hatipoğlu et al. 2010 İlbir Mountains study.
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Gem Article
Why Zultanite Has No Secondary Locality
Diaspore is geologically common — it occurs in every major bauxite deposit on Earth. Gem-quality color-change diaspore is not, because the simultaneous geological conditions required (slow metamorphic crystallization plus the specific iron-to-chromium…
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Gem Article
Pleochroism vs Color Change in Zultanite — Two Optical Effects, One Crystal
Pleochroism and color change are two distinct optical phenomena that both appear in diaspore. Pleochroism (different colors visible from different viewing angles in the same illuminant) is caused by the orthorhombic crystal's…
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Glossary Term
Bauxite
A sedimentary rock composed primarily of three aluminum hydroxide minerals: gibbsite, böhmite, and diaspore. Bauxite is the world's primary aluminum ore. Most bauxite-hosted diaspore occurs as fine crystals of no gem interest.…
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Glossary Term
Metamorphism
The process by which existing rock is transformed by heat, pressure, or chemically active fluids without melting. The İlbir Mountains diaspore deposit formed from regional metamorphism of bauxite-rich sediments under conditions favoring…
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Gem Article
The Absorption Spectrum of Color-Change Diaspore
Color-change diaspore from the İlbir Mountains shows a characteristic UV-Vis-NIR absorption profile dominated by trace iron (Fe³⁺ band near 425 nm) and trace chromium (Cr³⁺-related broad absorption centered near 580 nm). Transmission…
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Gem Article
Metamorphic Geology of the İlbir Mountains — Zultanite Source Formation
The İlbir Mountains diaspore deposit formed through regional metamorphism of bauxite-rich sediments under temperatures of approximately 350–450°C and pressures of 3–5 kbar — conditions in the greenschist-to-lower-amphibolite facies. The slow crystallization in…
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Education Article
How to Identify Zultanite — A Five-Step Field Guide
Identifying Zultanite (color-change diaspore) requires five tests: (1) check for color change between daylight and incandescent illuminants, (2) verify trichroic pleochroism with a dichroscope, (3) measure refractive index 1.682–1.752 with strong birefringence…
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Gem Article
The History of Color-Change Diaspore — From Haüy to the İlbir Mountains
Color-change diaspore — the mineral species — was first described by René Just Haüy in 1801 in the Mineralogie volume of his foundational works. Gem-quality material from the İlbir Mountains of southwestern…
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Glossary Term
Alexandrite Effect
Alexandrite effect (also called color change) is the appearance of distinctly different colors in the same gemstone under different illuminants — typically daylight versus incandescent or candlelight. Named after the gemstone alexandrite,…
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Glossary Term
Biaxial Positive
An optical character of anisotropic gemstones where light travels at three distinct refractive indices (nα < nβ < nγ) along three principal optical directions, with two optical axes along which light behaves…