![]() The gem below shows no chromium fluorescence under longwave UV light, but it does show a strong red reaction under the Chelsea filter. This quenching phenomenon is rarely recognized in gemology. Instead, vanadium V3+ quenches fluorescence, just as iron quenches fluorescence in other types of gems. Unlike chromium, vanadium V3+ does not cause red fluorescence under longwave UV light in gemstones of any kind. No chromium absorption peak at 680nm is visible with a spectrometer. By testing the dark green Tourmaline pictured below using a Chelsea filter, UV flashlight and spectrometer, we have determined that this gem is a typical Chrome Tourmaline colored primarily by vanadium. Vanadium seems to have no quenching effect on Chelsea filter reactions. It is probable that the dark blue portion with susceptibilities above 500 is composed primarily of Schorl (a separate species) rather than "Indicolite" (Elbaite species).Įven though vanadium is the dominant chromophore, chromium within all Chrome Tourmalines we have tested is detectable as red color under a Chelsea filter. Measurements taken from left to right at 8 different points along the length of the 24mm bar show a ten-fold decrease in magnetic susceptibility from SI 781 at the dark blue end to just SI 78 at the colorless end due to progressively lower iron content. Progressive changes in magnetism can also be measured within this bi-color Tourmaline. Lower concentrations of iron (blue color) result in lower densities along the length of the gem, accompanied by lower refractive indices. These unusual changes within a single gem are due to variations in iron content. The refractive index of this gem starts high at 1.63-1.655 on the dark blue end and decreases to the typical Elbaite range of 1.62-1.64 on the colorless end. The body color changes from opaque dark blue on the left side to colorless on the right side of the bar. The 3.23ct bi-color Indicolite Tourmaline bar pictured below is quite interesting, so let's take a close look at its identifying characteristics. However, for the sake of simplicity we will assume that most brown Tourmalines shown below are Dravite. Some of the brown Tourmalines in our study have been identified by Raman spectroscopy as Elbaite. Elbaite, Liddicoatite and Uvite gems can also be brown, presumably due to the same iron-titanium charge transfer process. Their color and lack of magnetic attraction are not specific to the Dravite species alone. The 3 brown Tourmalines pictured below show an Inert (Diamagnetic) response to an N52 magnet. Iron ions in brown Tourmalines are often not magnetically detectable because only very low concentrations of iron are needed to create color through the iron-titanium (Fe2+-Ti4+) charge transfer process. The iron is derived from mixing with Schorl. Another example is brown and orange Tourmaline, as discussed below:īrown and Orange: Dravite is typically a brown or orangey brown Tourmaline colored by iron to titanium charge transfer (Fe2+-Ti4+). Some Dravite and Uvite gems are also black. For example, not all black Tourmalines are Schorl. The transition metal impurities and charge transfer processes that appear in one species may also appear in other species. Gems of different species can have the same color. Different species generally can’t be distinguished from one another by their color or magnetic responses or magnetic susceptibility measurements. ![]() Tourmaline species are classified according to chemical compositions that are often unrelated to color or magnetism. 2- About the Magnetic Susceptibility Index Garnet pg 14- Gem Garnet Classification System.Garnet pg 13- Distinguishing Between Garnet Species and Varieites.Garnet pg 12- Ugrandite Garnets: Grossular Garnet. ![]()
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