Lab Notes Gems & Gemology, Winter 2013, Vol. 49, No. 4

Spinel Submitted as Diamond

Martha Altobelli, Paul Johnson, and Kyaw Soe Moe

This rough spinel, submitted as a diamond, displayed trigons oriented in the direction opposite that of the crystal face.
Figure 1. This rough spinel, submitted as a diamond, displayed trigons oriented in the direction opposite that of the crystal face. Photo by Joshua Balduf.
Diamond simulants remain in wide circulation and are a common sight in the GIA laboratory. Even so, it is rare that we encounter a natural gemstone masquerading as a colored diamond. The New York laboratory recently received a 0.50 ct rough sample with a remarkably saturated Fancy Intense to Vivid pink color (figure 1) for a Colored Diamond Identification and Origin Report. The stone displayed octahedral crystal form with numerous trigons on its crystal faces. These triangular etch pits, caused by chemical dissolution, are common characteristics of both natural diamond and spinel but have also been observed in flux synthetic spinels (M.S. Krzemnicki, “Trade Alert: Flux grown synthetic red spinels again on the market,” SSEF Newsletter, October 14, 2008, p. 3). When found on octahedrons, trigons point in the direction opposite that of the crystal face (again, see figure 1). Figure 2 shows a distinctive conchoidal fracture amid the trigons, a feature alien to diamond. This feature, along with the sample’s striking color, indicated something other than diamond.

In addition to trigons, the spinel displayed a conchoidal fracture (see arrow), a feature not found in diamond. Magnified 40x.
Figure 2. In addition to trigons, the spinel displayed a conchoidal fracture (see arrow), a feature not found in diamond. Photomicrograph by Martha Altobelli; magnified 40x.
The high-resolution UV-Vis spectrum displayed emission bands and absorption peaks not seen in natural diamond. Closer examination of the photoluminescence spectrum affirmed chromium peaks at 685, 687, 689, and 700 nm. Chromium causes the red/pink color in spinel but is not present in diamond. The narrow bandwidth of the 685 nm peak, along with the presence of the ~533 nm band in the UV-Vis spectrum, proved this was an untreated stone (S. Saeseaw et. al., “Distinguishing heated spinels from unheated natural spinels and from synthetic spinels: A short review of on-going research,”  www.giathai.net/pdf/Heated_spinel_Identification_
at_May_25_2009.pdf). Other observations included a specific gravity of 3.45–3.48, consistent with spinel, as well as medium to strong red long-wave and short-wave UV. Though submitted for a diamond report, this stone was actually a natural pink spinel.

This finding serves as a reminder of the importance of both standard gemological techniques and advanced spectroscopy in identifying gem materials.

Martha Altobelli, Paul Johnson, and Kyaw Soe Moe are researchers at GIA's New York laboratory.