Ametrine

Bolivia

Here is a map (20 Kbytes) showing the approximate location of the ametrine mine.

The ametrine occurs in veins in a dolomitic limestone. Crystals of Bolivian ametrine (29 K) range from 10 cm to 30 cm in length with diameters ranging from 4 to 12 cm. The interior of the crystals as seen in a slice of Bolivian ametrine (55 K) show the typical sector zoning. Another slice of Bolivian ametrine (55 K) shows the same sector zoning.

Inclusions of the fluid (37 K) from which the crystals grew can be found in most of the crystals. A small gas bubble can be seen in the center of the largest (0.4 mm wide) fluid inclusion.

In a new area of the mine recently opened (1999) called Pozo Rico, a 4 x 3 x 1 meter cavern of crystals was discovered.  Here are two pictures of the large pocket provided by the company. Note the miner for scale:  85 K,    79 K.

Here are other images concerning the mining of ametrine in Bolivia.

• The mining camp (43 K). The mine is located in the hill in the background. This picture was taken at an usually dry time, so the trees on the hill have little foliage.
• The entrance to the mine (60 K). The dark area just to the left of the grass hut is the opening to the mine. The rocks in the foreground are waste from the mine. The grass hut contains ventilation equipment for the mine.
• The sorting hut (45 K). Here is where the production is washed and sorted.
• A stock of large crystals (36 K) awaiting shipment at the mine.
• Many crystals are trimmed and only the gem quality interior portions are saved. Here is a 5 cm clear interior portion (14 K) of a crystal.
• Stockpiled bags of trimmed crystals (30 K) in Puerto Suarez, Bolivia, awaiting shipment to cutting centers.

India

Brazil

Synthetic ametrine is now produced in limited quantities in Russia. The original details of the development of bicolored quartz were reported by Balistky and Balistkya (1986). Dr. Balistky (26 K) developed the process for producing synthetic ametrine (16 K) at the Institute for Experimental Mineralogy (32 K) in the Chernogolovka Science Center, Russia, northeast of Moscow.

Commercial production takes place in a factory (13 K) at Alexandrov, Russia, in large hydrothermal vessels (23 K). The crystals are grown on racks of rectangular seed crystals (11 K). As initially grown, the crystals (49 K) crystals do not have the amethyst color. It will be developed in a later step. The shape of the seed results in a final crystal which has a different morphology from the natural material. In synthetic ametrine (16 K), the colorless seed crystal can be seen in the center of the crystal running its length from left to right. The citrine is in the interior and amethyst is at the rim of the synthetic ametrine crystal (12 K). Here are slabs from a crystal (8 K) which show the crystal as grown and after the outer zones are converted to amethyst with ionizing radiation from cobalt-60.  The interior of these crystals, seen in a slice of synthetic ametrine (26 K) , show the sharp demarcation between colored zones.

Crystals, grown experimentally on a seed of a different orientation may have different arrangements of the colored sectors as seen in this slice looking down the c-axis (9 K).  This compares to the arrangement of colors seen in a slice looking perpendicular (8 K) to the c-axis.

The crystals of synthetic ametrine are now available on the international market and are fabricated into carvings and faceted gemstones (7 K) and jewelry items.

Amethyst color develops in a two step process. First, individual Fe³⁺ions replace Si⁴⁺ ions in the quartz structure. Quartz with only these ions is nearly colorless. To develop the amethyst color, the crystal must be exposed to ionizing radiation to oxidize the iron to the 4+ state. In nature, gamma rays from the naturally occurring isotope potassium 40 (⁴⁰K) are probably the most important source of the radiation. Ametrine crystals found on the surface in sunlight have lost their color in the sectors which were originally amethyst. As is the case for all amethyst, the amethyst color center in ametrine is somewhat photosensitive and will be lost upon prolonged exposure to bright light.

The association of amethyst color to radiation is easily proven through the synthesis of amethyst in the laboratory. The development of amethyst color in synthetic ametrine after irradiation (8 K) is a case in point.

The growth of ametrine in the laboratory demands that the oxidation state of iron be carefully controlled and also that the rate of growth falls within a critical set of conditions. Quartz can also be grown under different conditions which result in the incorporation of Fe²⁺.  In quartz, Fe²⁺ causes a green color.  It is even possible to prepare crystals with iron in all three oxidation states in a single crystal as this slice of a Russian synthetic crystal shows.

The most complete reference to the Bolivian ametrine is:

The bicolor in synthetic Russian ametrine is described in:

Commercial synthetic ametrine production is briefly described in:

A detailed description of the production and properties of synthetic ametrine is in:

A more general technical discussion and review of color in quartz is found in: