Corundum Color: Vanadium, Chromium & Iron
The part played by vanadium in the colouration of gemstones often echoes that of chromium since trivalent vanadium absorbs in similar areas of the visible spectrum. The size and symmetry of the vanadium site is critical and this is one reason why vanadium-bearing minerals show a wide variety of colours (Schmetzer, Absorption spectroscopy and colour of vanadium (3+) – bearing natural oxides and silicates, N. Jb. Mineral. Abh. 144, 73–106, 1982 deals with this question). In corundum, vanadium causes the unmistakable (unless you have amethyst in mind) slate to purple colours of the material so often offered as, or at least confused with alexandrite. This is a flame-fusion product and shows a very sharp though not necessarily strong absorption band in the blue at 475 nm. This band is diagnostic for the material. corundum color change sapphire from Myanmar has been reported to show a vanadium spectrum.
A corundum color change sapphire from a recently reported source in the far south-west of Tanzania showed bluish green in daylight and reddish brown in incandescent light. Stones showed red though the Chelsea colour filter but did not respond to either type of UV. Both chromium and vanadium were found to be present. Nine out of ten specimens tested showed the true alexandrite effect, which is caused by the centering of the broad absorption band near 585 nm (550 nm in ruby). Narrow lines are also seen in the red, similar to a chromium spectrum. The vanadium spectrum is extremely rare in natural corundum. Mauve and purple sapphires will usually show elements of both chromium and iron absorption spectra. Since blue sapphire needs iron for a blue colour to be produced, any response to UV or stronger radiations would be surprising: one or two examples of blue sapphire with significant chromium content have been recorded, these specimens showing some orange or reddish glow under LWUV, such examples are hardly typical. Similarly some heat-treated blue Sri Lankan sapphires have been found to respond with a weak blue to SWUV. These specimens have contained colourless areas which have given rise to the response, the areas corresponding with growth structures. Most sapphires are inert under X-rays, except the Sri Lankan, Montana and some Indian (Kashmir) stones which may show a dull red or yellowish
It has been reported that under bombardment by cathode rays (fast-moving electrons), Kashmir sapphires show a greenish blue glow, Myanmar stones show a strong dark purple, Thai stones show a weak dull red and sapphires from Sri Lanka a vivid red. The iron-rich green and yellow sapphires show no luminescence of any kind, but yellow and orange stones from Sri Lanka show a strong apricot-yellow glow under UV, X-rays and gamma rays. The cause of this particular luminescence is not known. Such stones when bombarded by X-rays turn to a rich topaz colour, however weakly yellow they were originally. This colour is not permanent and reverts on exposure to about 3.5 hours’ sunlight or quickly when the stone is heated to about 23 T. Colourless sapphires may also suffer this change of hue after irradiation, but the shade of yellow attained is usually lighter; and further some blue sapphires will change to a dirty amber colour.