Intriguing and mysterious, opal stone fascinates, on the one hand because of the multitude of colors and splinters it may have, but also because it remains a stone difficult to pin down by scientists and gemologists .

It is surely one of the few gemstones still being studied and the subject of theses to understand the chemical and granular composition, explain its lights and its mechanism of formation.

Noble opal ring

Silver ring with noble opal custom made

Opal ring

Gold ring with fire opal and diamond

While scientific knowledge and technical means make it possible to produce diamonds, sapphires and synthetic emeralds in the laboratory, the industrial reproduction of synthetic opals remains difficult and the copies are easy to differentiate from natural opals.

The opal of fire is remarkable among all opals by its color ranging from yellow to dark orange almost red.

Mexico is the main producer of fire opal, found in volcanic areas of the country.

Fire opal is an emblematic stone of Mexico because it was already used by the Aztecs with jade and turquoise for the making of symbolic artifacts and jewelry.

fire opals

This photo shows the rich variety of colors that opals from Mexico can take.

Among the opals in this photo are the three types of opals used in jewelry : common opals, fire opals and noble opals characterized by their play of lights.

Index

Composition and formation of opals : water stones

Opal differs from other gems in two major points : it's not a crystal and has no precise color.

It's an amorphous or poorly crystallized silica that contains water.

Moreover the formula of its chemical composition: SiO2, nH2O clearly shows that the water content of opals is very variable, it can range from 1 to 21% for extreme values and remains between 4 and 10% in general.

The water content of an opal can be an important factor in its appearance.

Thus, an opal exposed to heat can dry and lose its appearance (phenomenon called "bleaching" or "destabilization", there is no known rule, it varies depending on the stones and extraction sites).

Likewise, Wello opals (originally from Ethiopia) once wet become transparent and lose their appearance as they recover when they regain their optimum moisture content for the play of light.

Welo Ethiopian opal

Welo opals, noble opals from Ethiopia

Also, hydrophane opals are known to be opaque opals without iridescence when dehydrated, but show color patches after rehydration in a fresh water bath.

Nevertheless, they are not considered as noble opals.

Mexican fire opal

Mexican fire opal characterized by its color and transparency. This one, a raw fire opal, is 22 carats and was extracted from a mine in the Magdalena region.

Unlike crystalline gems forming clusters, opals are formed in the crevices of the rock, thus marrying the cavity of the rock.

This explains the complex shapes taken by opals released from their rock matrix.

Composition

The chemical composition of the opal, as well as its mineralogical structure, influence the behavior of the light passing through the stone.

There is an infinite number of chemical and structural combinations, thus explaining the vast diversity of appearance of opals.

If the opal of fire stands out among all the common opals because of its transparency and its color, it is the noble opal which is the most appreciated of the world of the jewelry.

Its light effects called "color games" or "iridescence" are sought for the making of jewelry unparalleled.

noble opal noble opal noble opal

 

However, a fire opal can be noble if in addition to its color of fire we can observe an iridescence.

It is then called noble fire opal, much sought after, as you can imagine, which is the finest opal stone that Mexico can produce.

This is possible because the color of the stone and the iridescence each have their own origin.

The color in the mass of an opal comes from colored mineral inclusions while the play of light is due to the diffraction of light caused by an ordered network of silica spheres constituting the stone.

The creation of this ordered network depends entirely on the mechanism of opal formation.

Big Mexican fire opal

Raw fire opal stone 

The previous photo allows to see the color games of this opal that vary from yellow, orange to green.

As a result of the diffraction of the incident light on the ordered network of silica spheres, these color schemes vary in position and hue according to the orientation of the light.

This stone also offers blue and red lights that do not appear in this picture.

Formation

In general (because there are exceptions /0: Austrian and French opals) the opals are formed either in sedimentary medium or in volcanic environment, the most typical of which are respectively Australia and Mexico.

Opal is a secondary element of alteration, which means that it is formed from the dissolution of other phases and re-precipitation of the chemical elements mobilized.

Once translated, this means that an opal results from a dried soup, a soup in which the chemical elements of the surrounding environment have mixed.

Custom jaguar brooch with fire opal stone

Silver brooch with fire opal made in our jewelry workshop (fr)

The mechanism of opal formation meets these criteria:

  • Presence of very siliceous rock with very little other elements
  • The presence of holes or porosities
  • Alteration of rock by water
  • The presence of an impermeable layer, keeping the water in the presence of the rock
  • Gradual drying of the water + silica mixture

It must be added that a source of heat is also involved in this training process.

Two mechanisms are known from gemologists: the first is a cooling from 170 ° C, hot source of volcanic or hydrothermal type as is the case in Mexico.

Or cooling from 45 ° C, hot source due to tectonic movements, as was the case in Australia during the formation of Australian opals.

Opal mining in Mexico

The extraction of fire opals or noble doesn't differ from that of common opals.

It's indeed when we release the opal of its gangue of rock that we discover the type of opal we are dealing with.

Its extraction is manual, rarely using machine tools (which could damage the opal) and consists (at least in Mexico) of fragmenting the rock by blasting and then manually crushing the pieces to find the light bursts that betray the presence of opals in the rock.

Opal mine Mexico

View of a Mexican opal mine near the municipality of Magdalena in the state of Jalisco

If the exploitation of the opal in Mexico dates back to the Aztecs, the extraction sites fell into oblivion during the Spanish conquest.

It was not until the 19th century that the fortuitous discovery of opals revived its exploitation.

Currently, opals are found in the Mexican states of Querera, Guanajuato, Chihuahua, San Luis Potosi, Hidalgo, Jalisco, Nayarit, Guerrero, Michoacan, Oaxaca, Durango, México, Tlaxcala, Coahuila, Puebla, Zacatecas, or 16 of the 32 entities federal government.

Which explains why Mexico is the second largest producer of opal after Australia.

The map below shows the distribution of Mexican states producing opals. Not all of them produce fire opals or noble opals. The states of Queretaro and Jalisco appear in orange.

Opal mining area Mexico

All these states don't produce the same quantities and qualities of opals.

The state of Queretaro remains the cradle of Mexican opal where it was mined in 1840, producing most of the fire opals of the country, while the state of Jalisco produces most of the noble opals of Mexico.

Raw fire opals

Noble opal in its gangue on the counter of a lapidary workshop in Magdalena. The angle of the photo and the flash can only see part of the reflections of the stone, mostly blue, while the stone reflects green and red reflections.

Raw fire opal being cut

The same stone being cut

A few green sets of light can be seen on the upper part of the stone.

The cut phase affects the color play of the stone, this is due to the use of water for polishing so as not to overheat the stone that is fragile could split.

The opal takes again its color and its games of light afterwards.

The color of the rock containing Mexican volcanic opals is characteristic of opal rock veins in the Magdalena region.

Opals in Mexico history

Archeological studies have shown that the Aztecs knew the opals and used them for making jewelery and ceremonial utensils.

The opals they used came from mines located in the state of Queretaro and they named opal Vitzitziltecpal, which means "hummingbird" in reference to the many colors that can take the plumage of the hummingbird.

The most famous and probably the most beautiful Mexican opal is the one exhibited at the Gainger Field Museum in Chicago.

This 35-carat fire opal adorns a brooch depicting the God of the Aztec sun.

It is an exceptional piece for more than one reason, on the one hand it is an exceptional jewel and especially an Aztec archaeological remains dating back 500 years.

Aztec fire opal

Price of fire opals

Fire opals are fine stones, but their price may be close to that of certain precious stones.

We can find them cut in cabochon or with facets. Their price will depend on their size and the intensity of their color.

Prices range from 80 euros per carat for the cheapest to 500 euros per carat for the most beautiful.

Where to buy fire opals ?

Here are some places where you can shop online : 

To go further in the knowledge of opals

France has a team of experts on the theme of opal stone, team working at the faculty of Nantes and coordinated by Dr. Emmanuel Fritsch.

Although his studies are not available online, you can get them by contacting the university.

For further :

October 03, 2019 — Hugo Maherault

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