A natural, untreated sapphire’s hue comes strictly from its impurities. Corundum forms in igneous and metamorphic rocks. The more pressure the corundum receives over time, the clearer the sapphires are when mined. All sapphires are corundum, but not all corundum develops into sapphire. The chemical composition of corundum is Al2O3 (aluminum oxide). Pure corundum makes for a clear, colorless sapphire, while traces of other elements provide hue. It’s important to note that some sapphires present different color bands or areas within a single crystal.
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. Yogo sapphires, which come from Yogo, Montana, present a cornflower hue resulting from a combination of iron and titanium impurities. Violet sapphires contain vanadium (V).
If only iron makes it into the corundum, the resulting hue of the stone will be pale yellow or green. Iron and chromium (Cr) present together in corundum makes a sapphire yellow or orange. Pink sapphires contain chromium; if they contain enough chromium to make them brilliantly red, they are called rubies. Sapphires in the pinkish-orange range of color are called padparadschah (pronounced padpa-rad-scha), contain chromium and iron impurities and are found in Sri Lanka.
Going further into the science of a blue sapphire’s hue, intervalent (or cooperative) charge transfer is the process responsible for the color seen in these stones. This means that an electron is transferred from one transition-metal ion to another, producing a significant amount of coloration with a small amount of impurity (.01% of titanium and iron). At the same time, for corundum to be red enough to be a ruby, it must have a content of 1% chromium.
That still isn’t the end of understanding sapphire coloration. Many sapphires are treated. Low-grade enhancements include filling of cracks with glass or dyed wax; or they may be treated with beryllium or titanium and heat, resulting in false color that does not penetrate the entire stone. One or more heat treatments of a stone for removal of inclusions and enhancement of color is perfectly valid and acceptable. Heating a stone can reduce secondary colors and boost the primary color of a sapphire. Heat treatments may require temperatures as low at 400 degrees F or above 1700 degrees F to achieve desired results.
The range of color in natural sapphires is an expression of the gem’s impurities, and each stone is unique. Knowing how these jewels acquired their hues makes it all the more enjoyable to own one.
