Synthetic Color-Change Wakefieldite
Author AA, a gemstone cutter and collector of rare synthetic gems, recently submitted one rough and one faceted stone to GIA for scientific examination. The material, sourced from RG Crystals in Bangkok, was stated to be Czochralski-pulled, neodymium-doped yttrium orthovanadate, which is commonly used in laser applications. It can also form naturally as the mineral wakefieldite-(Y), with an ideal chemical formula of YVO4. His first observation was a striking color change from pink or purple to a blue or even green color under various lighting conditions and viewing angles. The finished gemstone (figures 1 and 2) displayed strong fire due to the material’s very high dispersion.
Wakefieldite is a rare tetragonal mineral belonging to the xenotime mineral family with an ideal general formula of (REE)VO4, where REE = Y, La, Ce, and Nd among other rare earth elements. Its general physical and optical properties include a hardness of 5, specific gravity of 4.25, uniaxial + optic sign with refractive indices of nω = 2.000 and nε = 2.140, birefringence of 0.140, and dispersion of 0.084. With prominent, perfect cleavage and relatively low hardness, the material is difficult to polish. Natural wakefieldite occurs as very small, non-gem crystals unsuitable for faceting; Czochralski-pulled materials represent the only examples of faceted wakefieldite. In fact, this was the first faceted synthetic wakefieldite seen in a GIA laboratory. Chemistry and visible absorption spectra were collected to quantitatively study the striking color-change behavior.
A polished wafer with 10.6 mm thickness was analyzed. Laser ablation–inductively coupled plasma–mass spectrometry revealed the material was composed of mostly yttrium and vanadium, with 2.2% neodymium by weight (0.03 per formula unit). Since Y3+ does not produce color on its own, the color must be derived from the addition of Nd3+ ions substituting for Y3+ ions in the crystal lattice. Figure 3 shows polarized absorption spectra in the visible range collected with an ultraviolet/visible/near-infrared spectrometer for both the ordinary ray (o-ray) and extraordinary ray (e-ray). The expected color was calculated from the absorption spectra based on the formula described by Z. Sun et al. (“Quantitative definition of strength of chromophores in gemstones and the impact on color change in pyralspite garnets,” Color Research and Application, Vol. 47, No. 5, 2022, pp. 1134–1154).
As shown in figure 4, the material exhibited a general pink to purple to violet color in most illuminants except the Eiko warm fluorescent light and the o-ray of CIE standard illuminant F10, which represents a typical cool fluorescent light. In fact, the color-change behavior noticed by author AA could be replicated when comparing illuminant F10 to any of the other illuminants such as daylight or incandescent light. Switching between warm incandescent light and actual cool daylight did not produce a dramatic change of color. The emission spectrum of F10 fluorescent light is mainly composed of sharp peaks at 435, 545, and 610 nm (figure 3). Therefore, the color of the synthetic wakefieldite crystal under illuminant F10 is only determined by the small absorption features at ~435, 541, and 610 nm of the Nd3+ ions. Broadband emitters such as incandescent light and actual daylight (D65 in figure 3), on the other hand, do not have these sharp emission features, so the color was determined by the transmission windows in the spectrum (440–460, 490–500, 550–570, and 620–670 nm), which remain constant when switching between these illuminants. This was confirmed by observation of the stone in outdoor conditions, which produced the same pinkish purple color seen in incandescent light. The use of warm fluorescent light (Eiko fluorescent light in figures 3 and 4) with color temperature of 2800K also reproduced the greenish blue to blue color seen in cool fluorescent light (F10). It is also worth noting that the absorption features of Nd3+ ions at 541 and 610 nm were strongly dependent on the polarization of the light, resulting in a strong pleochroism (grayish purple for the o-ray and green-blue for the e-ray), only observed under F10 illuminant (figure 4).
The color panels under two LED illuminants in GIA’s standard viewing box were also calculated in figure 4. The two LED lights, warm and cool, closely imitate the CIE standard illuminants A and D65. The two sets of color panels (A = LED warm pair and D65 = LED cool pair) are extremely close to each other based on their L, C, and H (lightness, chroma, and hue) color coordinates. This study also illustrates the consistent results of GIA’s standard viewing box in grading gemstone color.
