Ametrine is a rare bicolor variety of quartz that naturally combines the violet color of amethyst with the yellow-to-orange color of citrine in a single crystal. Most gem-quality ametrine comes from a remote deposit in Bolivia, where changes during crystal growth produced its distinctive color zoning.
When ametrine first appeared on the modern gem market, its unusual combination of colors caused considerable debate over whether the material was natural, synthetic or treated. Today, both natural and laboratory-treated ametrine exist. Understanding how this unusual quartz forms helps explain why its sharply divided colors—and the geometric patterns they create when cut—make it such a distinctive gem.
Ametrine Key Takeaways
- Ametrine is a bicolor variety of quartz that combines purple amethyst and yellow-to-orange citrine in the same crystal.
- Bolivia is the primary source of gem-quality natural ametrine, from a remote deposit known for its distinctive color zoning.
- Ametrine’s colors are related to iron impurities in quartz and changes that occur during crystal growth.
- Natural ametrine can show sharp geometric boundaries between its purple and yellow color zones.
- Cutting ametrine across its crystal structure can reveal triangular color patterns and allows lapidaries to emphasize or blend the two colors.
- Ametrine can also be produced through heat treatment, creating material that resembles naturally occurring ametrine.
- Not all quartz from the Bolivian deposit is ametrine; much of it is colorless, amethyst or opaque quartz.
What is Ametrine?
The original ametrine crystals are natural and come from quartz veins or stringers in a metamorphic deposit in Bolivia. But we have also figured out how to produce ametrine through heat treatment, which means we can copy nature.
Ametrine is a unique form of quartz where two gem colors occur in the same crystal. It is a fun treasure to carve, facet, and use to create jewelry and gem carvings.
The introduction of ametrine into the modern gem market came as a surprise, although this unique Bolivian gem deposit had been known to the native peoples in the area for several hundred years. The problem is that the area where the deposit is located is one of the most remote parts of Bolivia. Historical records show the local Ayoreo tribe had found and prized this unusual bi-color stone more than gold.
Ametrine History and the Legend of Princess Anahi
When the Spaniards conquered Bolivia and portions of western South America, they were mainly seeking gold and silver. The ametrine area where the Ayoreo tribe lived was so remote that it was not even visited by Spaniards until the 1600s. Even today, it is not an easily accessible area.
Upon arriving in the region of the Ayoreo tribe, a soldier, Don Felipe de Urriola y Goitia, was among the group. Instead of finding gold and silver, all the Spaniards found were bi-colored stones that the locals held in high esteem. It was only when the tribe’s Princess Anhai fell in love with Don Felipe that an event happened that brought the gem to Europe.
The Princess was so in love with Don Felipe that she gave him her treasured necklace as a sign of her everlasting love. The necklace pendant was a lovely bi-colored stone. This gift gave rise to the “Legend of Princess Anahi and her love of Don Felipe.”
Not finding gold or silver, the Spaniards, including Don Felipe, were ordered back to Spain. This meant Princess Anahi had to stay behind, and that is when she gave the necklace to Felipe.
The stone, with its beautiful yellow and violet colors, was a symbol of her undying love for him. There are different versions of what happened upon Don Felipe’s departure. Distraught over her separation, either the Princess disappeared into the mine workings, or she just pined away.
When Don Felipe returned to Spain in the 1600s, he gave the bi-colored stone to the Spanish queen, introducing it to Europe.
What Causes Ametrine’s Colors?
Ametrine is found in Russia. But it is not of gem quality anywhere else, so it is not unique as a gem, but unique to Bolivia. The colors in the better stones occur in more or less equal amounts of violet amethyst and yellow citrine. Each colored area is distinctly separate from the colors alternating side by side in the same crystal.
Properly cut across the vertical or “C” axis of the crystal, the two colors are often seen as alternating triangular color sections, clearly showing the dramatic difference between the two chemically different forms of quartz. Today, skilled artists have developed cutting methods so these two colors can blend, to create a mixture of the two colors, giving the impression of yet other colors, adding to the enhancement of the stone.
But how do we explain the triangular alternating color patterns in the stone that you see when you cut across the long axis of the crystal? The yellow citrine color alternating with the violet amethyst color, with a sharp boundary between the two colors, is quite distinct.
Why Does Ametrine Have Two Colors?
Remember, quartz is hexagonal in crystal form, so a natural quartz crystal will have six more or less equal sides or prism faces. Each of the six faces on a perfect quartz crystal is topped by triangular termination faces that come to a point. Small modifying faces can change that termination pattern, but internally, the three horizontal axes control the internal triangular arrangement, which meets in the center of the crystal. When you slice ametrine across the vertical to obtain a wafer of quartz, the quartz wafer shows this alternating triangle of violet and yellow-colored sections. Why is this the case?
Initially, it was thought these were twinned crystals. When a quartz crystal develops, the atoms form a crystal lattice structure that is a tetrahedron. These stack in an offset, spiraling manner. Later studies showed that ametrine is not internally twinned, so the quartz is a single crystal, and some other mechanism is at work here. We now know those colors are the result of changes in the oxygen electron valence of the iron oxide, which is a common impurity in quartz. The valence shift, which causes the color variations in the oxygen, is due to temperature differences.
How Iron Creates Ametrine’s Color
During ametrine’s development, quartz crystals change in the temperature of the solution during the crystal’s growth. This caused changes in the iron oxide’s oxygen. Citrine, the yellow form of quartz, is Fe2O3. In amethyst, the iron oxide is Fe3O4, and any change in temperature during crystal growth can result in the iron oxide change resulting in a color change in the quartz. This is because temperature changes affect the outer orbital valence electrons in the iron oxide, and that determines if the quartz is citrine or amethyst.
Since ametrine is found in a metamorphic environment, any variations in temperature and pressure within the rock formation during crystal growth can vary the outer orbital electron count of iron oxide, thus varying the color.
Where Is Ametrine Found?
As mentioned earlier, there was quite a disagreement over ametrine when it first appeared in the late 1970s. This was resolved when someone visited the deposit and collected specimens from a productive vein. A decade later, superb specimens were being marketed as the mine came under development. The ten-year delay is explained when you consider the remoteness of the ametrine location. It could only be reached after an arduous land-water journey, but now by air.
The Gemological Institute of America in Carlsbad, California, has created a detailed report on ametrine and the mine site.
How Does Quartz Form?
To understand what was going on for ametrine to form, you have to understand something about how quartz forms. Like so many crystals, quartz forms from silica-rich solutions that are hot enough to dissolve and hold the silicon dioxide molecules. The problem with quartz and any mineral that forms from a hot solution is that the watery solution is non-discriminating. It dissolves and absorbs anything and everything it encounters.
Hydrothermal solutions, as they are called, are never pure. They are rich in a variety of elements and mineral molecules. Pure quartz molecules are simply silicon dioxide, but two elements are easily accommodated by the quartz molecule: aluminum and iron, which can replace some of the silicon atoms. This sets up conditions needed for quartz to have color.
Given the unavoidable impurities in hydrothermal solutions, our interest here is with the iron that is an impurity in quartz. In one form, the iron oxide molecules cause quartz to take on a yellow color. In another form, the iron oxide gives quartz a lovely violet color. How can that be? Under normal conditions, an iron atom has an electron pattern of 2-8-14-2. But that next to the last orbital can give up one or two of its fourteen electrons, which then enter the last orbital, joining the two already there. So, iron’s last orbital can hold three or four electrons, each causing a color.
That shifting takes energy, which is absorbed from the light entering the quartz.
Remember, the quartz veins in the mine have formed under heat and pressure during the metamorphism of the host rock. So, the heat energy from metamorphic action gives those transferring electrons the energy they need to shift, creating a color.
How Does Ametrine Develop Its Color Zoning?
But why the alternating yellow-violet colors? That is more difficult to explain. As the quartz crystals are developing, the internal geometry of the atoms may vary slightly, perhaps due to the presence of iron oxide. Iron atoms are a different size from silicon atoms, so they slightly distort the atomic structure of the silicon dioxide. That may be part of the cause of the color pattern. We do know that the number of valence electrons in each iron atom is important. Iron valence three gives citrine a yellow color. Iron valence four, in its outer orbital valence, causes the lovely violet color.
It takes more energy to achieve iron valence four than iron valence three. So, when the rock-forming process is ongoing, and the temperature varies higher and lower, that temperature difference results in one color, then the other forming, and the crystal becomes bi-colored.
The Unique Ametrine Deposit in Bolivia
But something else must be going on within the quartz crystals in this deposit to give the quartz the lovely geometric zoning of two colors. This zoning pattern only occurs in about one-third of the quartz mined here; the rest is colorless, amethyst or opaque quartz.
Once scientists figured out what caused the citrine-amethyst colors in this quartz, it didn’t take them long to replicate it in the lab. You can not tell the difference between natural and treated ametrine visually, but pricing would make it obvious. Natural ametrine is very costly, and understandably so. The site is still remote. Even the early Spaniards did not enter the region early during their invasion. Even today, with an airstrip nearby, it is still more than a three-hour flight to reach the site. The mine owner had to install an airstrip to get equipment in and product to the market.
Ametrine Frequently Asked Questions
What is ametrine?
Ametrine is a bicolor variety of quartz that contains both purple amethyst and yellow-to-orange citrine in a single crystal. Its two distinct colors are what give this unusual quartz its name and visual appeal.
Where is ametrine found?
The most important source of gem-quality natural ametrine is a remote deposit in Bolivia. The original ametrine crystals come from quartz veins or stringers within a metamorphic deposit.
Why is ametrine purple and yellow?
Ametrine’s purple and yellow colors are associated with iron impurities in quartz and conditions that change during crystal growth. Differences within the growing crystal produce the distinct color zones associated with ametrine.
Is ametrine natural?
Yes. Natural ametrine occurs in nature, particularly in Bolivia. Ametrine can also be produced through heat treatment, so both natural and treated material are found in the gem market.
Is ametrine a type of quartz?
Yes. Ametrine is a bicolor form of quartz. Its two colors correspond to the purple of amethyst and the yellow color of citrine.
What colors does ametrine come in?
Ametrine typically combines purple or violet amethyst with yellow to orange citrine. In well-defined crystals, the two colors can occur in distinct zones within the same quartz crystal.
Why does ametrine have triangular color patterns?
When an ametrine crystal is cut across its vertical axis, its internal color zoning can appear as alternating triangular sections of purple and yellow. The geometry is related to the internal structure and growth of the quartz crystal.
Is ametrine valuable?
Natural ametrine can be valuable, particularly when it displays strong, well-defined color zoning and comes from the limited natural supply. Treated ametrine is also available, and the two types can be difficult to distinguish visually.
Final Thoughts
Ametrine stands out among quartz varieties because two familiar gem colors occur naturally within a single crystal. Its combination of amethyst’s purple and citrine’s yellow, along with the geometric zoning revealed by cutting, makes each natural crystal a fascinating example of how conditions during mineral growth can influence color.
The exact processes responsible for ametrine’s distinctive color zoning are part of what makes this Bolivian quartz deposit so intriguing. Whether encountered as a natural crystal, a faceted gemstone or a carefully cut carving, ametrine offers collectors and gem enthusiasts an unusual look at the relationship between crystal structure, chemistry and color.
Love learning about unusual minerals and gemstones? Subscribe to Rock & Gem for more stories about Earth’s treasures, mineral science, collecting and lapidary arts. Story by Bob Jones.
