What is synthetic spinel?

Although not as widely used today as synthetic Corundum, synthetic Spinel is still one of the most common man-made gem materials, particularly blue synthetic Spinel. It appears frequently in class rings and costume jewelry. Blue, green and other colors of synthetic Spinel are mostly manufactured using the inexpensive flame fusion method by which the Spinel develops around an elongated boule. These flame fusion Spinels can be distinguished from natural Spinels by a slightly elevated refractive index due to a chemical variation involving higher aluminum content.

Blue synthetic Spinels of the flame fusion type are typically colored only by cobalt. No iron is added, and therefore they are diamagnetic, another property that distinguishes blue flame fusion Spinels from blue natural Spinels. When viewed under the Chelsea filter and also under longwave UV light, the light blue and medium blue gems pictured below left and center show a pale pink reaction due to low cobalt content, while the dark blue gem on the right appears bright red due to much higher cobalt content. None of the synthetic Spinels shown below contain enough cobalt to cause magnetic attraction.

One anomalous blue synthetic flame-fusion Spinel was found to show a weak magnetic response due to manganese (Mn2+). This gem has an unusual bright blue-green or "aqua" blue color, and fluoresces a strong yellow-green color rather than red under longwave UV light, indicating manganese content. The concentration of manganese is likely higher than the concentration of cobalt, overtaking the typical pink or red fluorescence of synthetic blue Spinel.

With the aid of a spectrometer, we detected a small amount of manganese in other light blue synthetic Spinels, but only this one sample contained enough manganese to cause green fluorescence under longwave UV light, along with magnetic attraction. All light blue synthetic Spinels we tested did fluoresce yellow under short wave UV light due to manganese.

The purpose of adding manganese to synthetic Spinel is not clear, but perhaps it brightens the blue color by blending in a subtle yellow-green component. This could be similar to what we find in "neon" or "aqua" blue Paraiba Tourmalines that are naturally colored blue by copper and perhaps modified slightly in color by manganese.

cobalt blue synthetic spinel rough

cobalt blue synthetic spinel rough

Blue Spinel gems manufactured by the slower and more costly flux-grown method have a vivid blue color due to high cobalt content. They are generally small in size, faceted from small synthetic crystals that are shaped like natural octahedral Spinel crystals, as opposed to the large rounded boule shapes of flame fusion Spinels. Because the major chemistry is the same as natural Spinel, the refractive index range is also the same. While the common flame fusion synthetic blue Spinels are almost always diamagnetic, flux-grown blue Spinels are weakly magnetic, most likely due to their higher cobalt content. This is the only gem material of any kind that we have found that shows magnetic attraction entirely as a result of cobalt. No iron or manganese can be detected.

Blue flux-grown synthetic Spinel could be mistaken for natural Cobalt Spinel, as both have the same strong blue color, the same refractive index, and both show weak magnetic attraction. However a moderate to strong magnetic response in any vivid blue Spinel would indicate iron content and natural origin. Fortunately, flux-grown blue Spinels are rarely found as a gemstones because they are time-consuming to produce, costly, and small in size compared to flame fusion Spinel gems. Blue flux-grown Spinels do not fluoresce as brightly under long wave UV light as do dark blue flame fusion synthetic Spinels, and we speculate that this is possibly because of concentration quenching produced by a higher concentration of cobalt.