宝石ニュース·インターナショナル Gems & Gemology, Summer 2000, Volume 36, No. 2

Ametrine with Layers of Smoky Quartz


Yuruty(ユルティ)鉱山産のスモーキー・クォーツ・アメトリン
Two parallel zones of smoky quartz are evident in these three ametrine specimens (27.3–299.7 ct) from the new Yuruty mine in Bolivia. Photo courtesy Michael S. Krzemnicki, SSEF.
Ametrine (amethyst-citrine) from Bolivia has been prevalent in the gem trade for many years (see, e.g., Gem News, Fall 1989, pp. 178–179, and Spring 1993, p. 53). At this year’s Tucson gem show, H. Marancenbaum of Steinmar Ltd. (Santa Cruz, Bolivia) kindly provided two rough fragments, three slightly polished crystals, and two fantasycut specimens of ametrine to the SSEF Swiss Gemmological Institute. Mr. Marancenbaum reported that these samples originated from the Yuruty mine, a new operation that is located in the same geologic unit (Murciélagos limestones) in Bolivia as the well-known Anahí mine (Vasconcelos et al., Gems & Gemology, Spring 1994, pp. 4–23).

An investigation of these Yuruty mine samples by the present contributor revealed an interesting new aspect to this bicolored quartz. In addition to the typical amethyst and citrine sectors (along the major and minor rhombohedral forms; again, see Vasconcelos et al., 1994), five of the samples also showed a similar pattern of two distinct layers of smoky quartz. These layers are oriented parallel to the minor rhombohedral form z {011 – 1}, within the citrine sector (see, e.g., figure 4). In the slightly polished crystals, the smoky layers were observed just below the surface. In this area, the color of both amethyst and citrine is rather pale. The two (broken) fragments that did not show smoky quartz layers were presumably derived from the central portion of a crystal; they did display more intense amethyst/citrine coloration.

The growth conditions of this unusual ametrine evidently facilitated the enhanced accommodation of aluminum impurities in the z sectors. Natural irradiation of these aluminum-rich zones would give rise to the smoky quartz color layers.

Approximately 30 tons of material have been produced at the Yuruty mine since February 1999, according to Mr. Marancenbaum. Large quantities of rough and cut amethyst, citrine, and ametrine from this new mine were available at this year’s Tucson show, and he anticipated that about 10,000 carats of cut material monthly could be expected in the near future.