Ametrine is a variety of quartz that contains both amethyst and citrine
sectors in the same crystal. Both amethyst and citrine are colored by small
amounts of iron (approx. 40 parts per million). Amethyst color develops
when iron-containing quartz is exposed to ionizing radiation. In nature,
gamma rays from the decay of potassium-40 are the most likely source of
ionizing radiation. The model currently accepted is that radiation oxidizes
Fe3+ to Fe4+. There is still uncertainty about the
site of the iron. Both interstitial sites in the c-axis channels, and the
silicon tetrahedral sites have been proposed as the site of the amethyst
center. Citrine color is from Fe3+. The
properties of the Fe3+ spectra suggest that the Fe3+
ions are aggregated and hydrated in clusters of unknown size.
The only significant source of natural ametrine is the AnahÝ mine,
in eastern Bolivia. The mine is operated by
Minerales y Metales del Oriente, S.R.L.,
Santa Cruz, Bolivia, and employs about 70 workers at the mine site.
It is the source of all the natural ametrine
currently on the world's market. In the early days of production, there
was much mis-information about the locality with Brazil and Uruguay frequently
being mentioned as the source of ametrine.
Here is a map showing the approximate location of the ametrine mine.
The ametrine occurs in veins in a dolomitic limestone. Crystals
ametrine 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 show the typical sector zoning. Another
slice of Bolivian ametrine shows the same sector zoning.
Inclusions of the fluid 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)
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: picture 1, picture 2.
Here are other images concerning the mining of ametrine in Bolivia.
The best quality production from the mine is used for faceted ametrine,
amethyst, and citrine. Some specimen material and ametrine slabs
are also produced. Mine run material is used for colorful carvings
The mining camp. 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
entrance to the mine. 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. Here is where the production is washed and sorted.
stock of large crystals 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 of a crystal.
Stockpiled bags of trimmed crystals in Puerto Suarez, Bolivia, awaiting shipment to cutting centers.
Minor amounts of citrine occur in some amethyst
from Hyderabad, India. The 5 cm wide crystal contains only
a minor amount of citrine color. Citrine color is found in few specimens.
In the magnified
image the view across is about 2.5 cm.
An occasional specimen of Brazilian
bicolored quartz has been found which contains both amethyst
and citrine zones. The magnified
view shows that it does not approach the quality of the Bolivian
material. The view is about 2.5 cm across.
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 developed the process for producing synthetic
ametrine at the
Institute for Experimental Mineralogy in the Chernogolovka
Science Center, Russia, northeast of Moscow.
Commercial production takes place in a factory at Alexandrov, Russia, in large hydrothermal
vessels. The crystals are grown on racks of rectangular
seed crystals. As initially grown, the
crystals 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, 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. Here are slabs
from a crystal 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, show the sharp demarcation between
Crystals, grown experimentally on a seed of a different orientation
may have different arrangements of the colored sectors as seen in this
looking down the c-axis. This compares to the arrangement
of colors seen in a slice looking perpendicular to the c-axis.
The crystals of synthetic ametrine are now available on the international
market and are fabricated into carvings and faceted
gemstones and jewelry items.
The color of ametrine
Citrine is a term applied to a variety of different quartzes that range from
yellowish green to orange brown. These colors have a number of different
origins. Some are due to changes brought about by exposure to naturally occuring
gamma rays and some are due to minor differences in the chemical composition.
The citrine color in Bolivian ametrine appears to come from the incorporation of
very small aggregates of Fe3+, most likely in the form of a
hydrous iron oxide. Microprobe analyses have shown that the orange-yellow citrine
sectors have, on average, about 70 ppm of iron compared to the amethyst sectors that have
iron concentrations that average from 20 to 40 ppm.
Amethyst color develops in a two step process. First,
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 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:
Vasconcelos PM, Wenk HR and Rossman GR (1994) The AnahÝ Ametrine
Mine, Bolivia. Gems & Gemology, 30, 4-23.
The bicolor in synthetic Russian ametrine is described in:
Balistky VS and Balitskaya OV (1986) The amethyst-citrine dichromatism
in quartz and its origin. Physics and chemistry of Minerals 13, 415-421.
Commercial synthetic ametrine production is briefly described in:
Balitsky VS, Makhina IB, Mar'in AA, Dorogovin BA, Shigley JE, Rossman GR
(1997) The first commercial synthetic ametrine from Russia ( Abstract
26th International Gemmological Conference, Idar-Oberstein, Germany, Sept.27-Oct.3,
A detailed description of the production and properties of synthetic ametrine
Balitsky VS, Lu T, Rossman GR, Makhina IB, Mar'in AA, Shigley J, Elen S,
Dorogovin GA (1999) Russian Synthetic Ametrine. Gems & Gemology 35,
A more general technical discussion and review of color in quartz is found in:
Rossman GR (1994) Colored varieties of the silica minerals. Reviews in Mineralogy,
last revised: 27-Apr-2012