Quench-Crackled Blue Synthetic Spinel
Recently, GIA’s Bangkok laboratory examined an interesting 9.75 ct blue oval mixed cut (figure 1). Standard gemological properties included an RI of 1.728, with strong anomalous double refraction using the polariscope. The sample’s Chelsea filter reaction exhibited a strong red transmission, and absorption bands at 540, 580, and 635 nm were seen in the handheld spectroscope. The specimen fluoresced strong red under long-wave UV radiation and strong bluish white under short-wave UV. These properties are diagnostic features of cobalt blue synthetic spinel grown by the Verneuil (flame-fusion) process.
Microscopic observation revealed tiny strings of gas bubbles and strong irregular graining, with many reflective fractures on one side of the sample (figure 2). The specimen appeared to have been heated and thermally shocked. This method, known as quench-crackling, typically involves the use of a dye to create a preferable color, but not in this specimen. While DiamondView imaging showed clear fractures similar to those found in quench-crackled treated material (figure 3), a PL spectrum of Cr3+ shifted from 685 to 689 nm was consistent with synthetic spinel (S. Saeseaw et al., “Distinguishing heated spinels from unheated natural spinels and from synthetic spinels,” GIA Research News, April 2009).
Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) was utilized for a trace element analysis. The results displayed high amounts of Co (up to 167 ppma), along with 1137 ppma Ti and 337 ppma Cr. Zn was found with a maximum of 6 ppma, and Ga was below the detection limit. This specimen contained higher amounts of Ti and Co than those previously reported in S. Saeseaw et al. (“Cobalt diffusion of natural spinel: A report describing a new treatment on the gem market,” GIA Research News, June 2015). We concluded that a quench-crackling technique was used to make the synthetic spinel appear more like a natural spinel. In such cases, standard gemological properties are useful for accurate identification.