If only iron makes it into the corundum, the resulting hue of the stone will be pale yellow or green. Yogo sapphires, which come from Yogo, Montana, present a cornflower hue resulting from a combination of iron and titanium impurities. Iron (Fe) and titanium (Ti) impurities must be present in a sapphire to give it the remarkable blue hue for which sapphires are best known. It’s important to note that some sapphires present different color bands or areas within a single crystal. Pure corundum makes for a clear, colorless sapphire, while traces of other elements provide hue. The chemical composition of corundum is Al2O3 (aluminum oxide). All sapphires are corundum, but not all corundum develops into sapphire. The more pressure the corundum receives over time, the clearer the sapphires are when mined. Corundum forms in igneous and metamorphic rocks. A natural, untreated sapphire’s hue comes strictly from its impurities.
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