The over-dark blue color that is often seen in blue Sapphires with high iron content is due to maximized iron to titanium (Fe2+-Ti4+) charge transfer, and also possibly to Fe2+-Fe3+ charge transfer. This additional iron may exist as cryptic (colorless) iron, or possibly become involved in iron to iron (Fe2+-Fe3+) charge transfer, which may modify blue color. In most natural blue Sapphires there is also some "surplus" iron, perhaps mostly Fe3+. This process is such a strong chromophore that only a trace amount of iron (too low to be magnetically detectable) is all that is necessary to create vivid blue color through Fe2+-Ti4+ charge transfer. Fe2+ loses an electron to become Fe3+, and Ti4+ gains the electron to become Ti3+. Iron is involved in the coloration of synthetic blue Sapphire as it is in natural blue Sapphire, so how do we explain differences in magnetic response? Through the process of inter-valence charge transfer, ions of iron (Fe2+) and titanium (Ti4+) interact with each other to cause blue color in Sapphire. 2- About the Magnetic Susceptibility Index
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