A Unique Omphacite Jade Pendant
In the past decade, omphacite jade with strong to vivid green color has increased in popularity. Especially noteworthy is the high-quality omphacite jade now coming from Guatemala. There are generally two types of omphacite jade. The first, commonly known as “black omphacite jade” (mo cui in Chinese), is typically opaque to semitranslucent and black to dark green. The second type contains higher chromium content and is usually made into cabochons, tablets, and pendants of 2–3 mm thickness to produce a strong green color and semitransparent appearance.
Recently, a carved pendant measuring 68.4 × 39.9 × 6.4 mm was submitted to the Taiwan Union Lab of Gem Research (TULAB) for identification (figure 1). The pendant was semitranslucent; its color appeared black when viewed under reflected light but dark green and yellow when viewed with transmitted light. Due to the pendant’s uneven surface, its refractive indexes were challenging to measure. The dark green part showed a refractive index of approximately 1.67, and that of the yellow part was around 1.71 (both by spot reading). To further confirm the mineral composition, Raman spectroscopy with 785 nm excitation was performed on the yellow and dark green areas. The spectra were compared with the RRUFF database (Lafuente et al., 2015, https://rruff.info/about/downloads/HMC1-30.pdf) in figure 2, and the results indicated that the main component of the dark green portion of this pendant was omphacite. However, the yellow regions were identified as vesuvianite, constituting approximately 30% of the entire piece. Omphacite jade displaying both dark green omphacite and paragenetic yellow vesuvianite are not common.
From a gemological perspective, this green and yellow pendant is undoubtedly an exceptional case, given the rarity of omphacite jade coexisting with vesuvianite. However, precisely calculating the amounts of omphacite and vesuvianite with destructive X-ray diffraction analysis was not possible.
