A Remarkably Large Clinohumite
Author: Gagan Choudhary and Chaman Golecha
(This article was first appeared in The Journal of Gemmology, Vol.30, No.5/6, pp 303-306)
Most commonly found only as small grains, rare larger Clinohumite crystals are sought by collectors; some are occasionally fashioned into bright yellow-orange gemstones. Though the mineral is known for a long time, gemmy crystals have been available in the international market only for the past two decades or so. Gem quality Clinohumite is known to occur in two important localities- the Pamir Mountains (at Kukh-i-lal, Sumdzin and Changin) in Tajikistan in association with spinel and the Taymyr region (Basin of Kotui River) in Siberia (O'Donoghue, 2006, p.400; Henn et al., 2001; Laurs and Quinn, 2004). Another find of Clinohumite has been reported in association with Spinel in Mahenge, Tanzania (Hyrsl, 2001). The authors encountered a large brownish orange gem clinohumite submitted for identification at the Gem Testing Laboratory, Jaipur.
Figure 1: Brownish orange clinohumite of 9.45 ct. Photo by G. Choudhary
The sample (figure 1) was an oval mixed cut measuring 15.13 x 10.41 x 7.78 mm and weighing 9.45 carats. It was transparent and had a bright brownish orange colour showing a strong resemblance to hessonite and spessartine garnets. The stone was slightly included with eye visible inclusions (again figure 1) and was reported to be from Tajikistan.
Using traditional gemmological equipment, the stone's properties were determined and are summarized in table 1. Refractive index and birefringence observed were consistent with the values mentioned in literature (e.g. O'Donoghue, 2006). The specimen exhibited a strong pleochroism with yellow, orange and brownish yellow being the three hues and under the short wave ultra violet lamp it displayed strong orange- yellow fluorescence with a surface related greenish yellow chalkiness (figure 2) while no reaction to long wave ultra violet radiations. The surface related chalkiness under SWUV reminded us of fluorescence commonly associated with brown glass coloured by uranium.
Figure 2: Strong yellow-orange fluorescence of the clinohumites under SWUV radiation; also note the surface related greenish-yellow chalkiness. Photo by G. Choudhary.
Table 1: Gemmological properties of tested Clinohumite weighing 9.45 carats
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