Large Euclase, Reportedly from Zimbabwe
By Meenu Brijesh Vyas and Gitiksha Khandelwal
A vivid blue cushion mixed-cut specimen measuring 15.24 × 9.04 × 9.74 mm and weighing 11.77 ct was submitted to the Gemological Science International (GSI) laboratory in Jaipur, India, for identification. When observed from the crown with the unaided eye in transmitted light, a beautiful blue color was seen (Fig.1a). However, a side view of the stone revealed it was mostly colorless with a narrow blue zone concentrated along the culet (Fig.1b).
Fig.1: This 11.77 ct cushion-mixed cut was identified as euclase. (a) It appears blue when viewed face-up, although a side view (b) shows the specimen is mostly colorless with a blue zone concentrated along the culet. Image @ Sandeep Kumar Vijay, GSI
The placement of this zone at the culet enhanced the visibility of blue color throughout the stone when viewed from the crown, similar to the effect that is sometimes seen in faceted blue sapphires. Gemological properties of the stone were recorded as follows: RI: 1.650–1.670 (birefringence 0.020); hydrostatic SG: 3.08; and inert to both long- and shortwave UV radiation. Observation with the gemological microscope revealed a cleavage trace (Fig. 12) and partially healed fractures (fingerprints’) associated with distinct two-phase inclusions (Fig.13), as well as dotted inclusions and tiny transparent crystals.
A strong doubling of facet edges was observed, consistent with the specimen’s birefringence and relatively large size. The stone’s properties matched those of euclase, a rarely encountered beryllium aluminium hydroxide silicate with the formula BeAl(SiO4)(OH). This identification was
confirmed with Raman spectroscopy. In addition, infrared spectroscopy yielded a spectrum that was also consistent with the euclase reference in our database. Chemical analysis with EDXRF spectroscopy using a Shimadzu EDX-8000 unit showed traces of Fe, consistent with the blue colouration of euclase (Stocklmayer 1998). The pure blue color of this specimen, combined with the distinct colorless and blue zoning, is consistent with a Zimbabwe origin (cf. Stocklmayer 1998). Euclase is rarely faceted as stones weighing more than 2.00 carat, making this 11.77 carat stone notable. By comparison, the next largest euclase documented in the literature to the authors’ knowledge is a greenish blue Colombian stone weighing 9.50 ct (Sauer 2006).
Fig.2: A cleavage fracture is seen near the girdle of the euclase. Image @ Sandeep Kumar Vijay, GSI
Fig 3: Left: ( a) The sample inclusion that appeared to be a laser drill hole. Right: (b) A laser drill hole in a natural diamond that reaches an inclusion for bleaching to improve face-up appearance. Note the resulting stress fracture and floret pattern due to the acid etching. Image @ GSI
References
- Sauer, D.A. 2006. Gem News International: Faceted blue euclase from Colombia. Gems & Gemology, 42(2), 173.
- Stocklmayer, S. 1998. Blue euclase from Zimbabwe – A review. Journal of Gemmology, 26(4), 209–218, https://doi.org/10.15506/jog.1998.26.4.209.