Fire Opal is one of the world’s most celebrated gemstones, and it is easy to see why. This mineraloid has a large variation in the size and arrangement of tiny silica spheres in opal. As a result, opal has the ability to exist in such a wide array of colors and patterns. When very small silica spheres make up the stone, the predominant color will be in the blue spectrum, resulting in Blue Opal. Alternatively, larger spheres of silica result in the warmer hues of orange and red displayed in Fire Opal, and are less common. Updated June 26, 2020.
What is Fire Opal?
Opal is an amorphous mineraloid that comprises hydrated silica (SiO2·nH2O). Opal lacks the structural organization necessary to form crystals. This mineraloid’s wide range of brilliant colors and dazzling iridescence captivates people from all around the world. And this has have been the case for millennia. In fact, the Ancient Romans called it “opalus” meaning “precious stone.” But the relationship between opals and humans trackback much further.
Opals are thought to have been mined several thousand years ago in East Africa, as indicated by archeological discoveries in that region. And the love of this geological marvel continued through the bronze age, into Ancient Greece and Rome, the middle ages, and into the present-day. In fact, there was an “opal rush” of the 1900s. The regions of Ethiopia are reportedly the earliest mining locations for Fire Opal, and currently remains an abundant source. Other major mining operations exist in Australia, Mexico, and the United States.
Is Fire Opal Expensive?
Opal certainly has the potential to fetch high prices in the international gemstone market. But its value can vary widely and depends on several factors. Specialists base the overall quality of opal on the type, color, and brilliance or brightness of that color. In fact, the most important consideration when determining value is the type of opal. This ranges from white opal, with a very light background, at $10 or less per carat, all the way to black opal, with its dark background and extreme rarity allowing prices to soar to $10,000 per carat. The colors within Fire Opal are important as well. For example, red is the rarest and most valuable color, and at the opposite end of the spectrum, blue is the most common and affordable color.
There are also market-driven factors such as worldwide supply-and-demand and publicity about certain stones. Additionally, the opal mines have variable production rates, which plays a strong part in the determination of opal prices around the world.
Are Fire Opals Rare?
In general, Fire Opal is rare. Precious opal is the result of a geological occurrence that is extremely rare. Even though silica is a very common mineral, the process of forming an opal that displays “play of color” does not happen very often. Opal miners estimate that up to 99% of all the opal they find is common opal. And of that 1% of opal displaying play of color, a tiny fraction is the highly sought-after black opal. Likewise, opals displaying shades of red are quite hard to come by. So, clearly, the price of opal ties directly to their rarity. Prospectors frequently uncover common opal which is of little value. On the other hand, precious opal is scarcer than diamonds. Especially rare are black opal, and precious opal set in fossils.
The Origins of Fire Opal
Fire Opals are named for their red, orange, and yellow coloration, and they may be precious or common. Most Mexican Fire Opals usually do not show the same play of color that is prominent in Australian Fire Opals. Rather, the Mexican varietal is sought after for its rich coloration. For instance, Mexican Opal often contains red, orange, yellow, or a combination of these colors. And these translucent displays of color can occur in patterns, seemingly dancing as if a fire flame were flickering inside. Thus, the name “Fire Opal.”
Typically, Australian Fire Opal does not display as rich of a fiery-base… But this is offset by a more fantastic play of color which often overlays a lighter coloration of stone with subtle splashes of fire. The almost solid, deep, red coloration that is possible with Fire Opals is the reason these gemstones are so precious. And red is the rarest of all the colors possible within an opal.
Fire Opal in Mexico
In the year 1835, excavations took place at the Mexican highlands for the first time in centuries, gifting humanity with flame-colored gems buried deep among the ancient volcanoes. Mexico’s historical mining of opals pre-dates even that of Australia. In fact, the national gemstone of Mexico the Fire Opal. This is where one can find the most significant deposits of that rare and specific varietal of opal with the fiery red glow. The Mayans and Aztecs revered the Fire Opal for its beauty, and it was incorporated into not only their artwork but also ritualistic ceremonies.
What is the Geology of Opal?
For opal to form, groundwater must fall slowly through subsurface layers that contain silica. As gravity causes the water to percolate through the silica-rich rock, a solution forms. And the solution is a combination of water and silica. The silica-bearing liquids then settle within a void, hole, or crevice, where the silica deposit maintains a formation for millions of years. As the water eventually evaporates out, the hydrated silica spheres (which will retain up to 20% water locked in their molecular structure) remain in place. And as this process continues, the layers of silica spheres build, and creates Fire Opal.
What are Mineraloids?
Mineraloids aren’t minerals, but mineral-like substances. For instance, Fire Opal is amorphous, meaning it does not possess a crystalline structure. While true minerals are very consistent in the arrangement and composition of their crystals. Unlike true minerals, opal, and other mineraloids, can vary slightly in the organization of their components. Other examples of mineraloids include pearls, amber, and obsidian. Most mineraloids, including opal, form at lower temperatures and lower pressures, such as those which exist in the shallow subsurface. Nonetheless, gem collectors categorize opal along with other minerals, even though it lacks the specific properties to truly and accurately be one.
As a result of their composition and the way they form, opals are not as hard as many other gemstones. In fact, on the hardness scale opals fall at 5.5 to 6.5. As a result, they are susceptible to damage by exposure to other harder surfaces, as well as fracturing within rapid changes in temperature or moisture.
A condition that occurs randomly and frustratingly in opals is “crazing”, which describes the formation of cracks both within the opal and exteriorly. Thus, gem workers sometimes store opals in water before cutting because this reduces the chance of crazing.
Upon removing fire opal from water, the risk of breakage ensues, and the problem exacerbates when the cutting and polishing process begins. Opals have no cleavage, nor the tendency to split along definitive and straight planes. And cleavage is a property in association with a crystalline structure. So conversely, when opal breaks, it does so along with a curve or irregular line.
Gem enthusiasts can purchase large quantities of rough opal directly from the miners. These are “parcels.” By properly assessing the rough stones, it is possible to gain insight as to the potential of the parcel. For instance, it’s ability to produce a certain quality and quantity of finished stones in the market.
Opal cutters generally prefer the use of a diamond saw to shape the rough opal into a smoother product. One challenge is to cut off the proper amount of excess material. Usually, designers aim to cut the common or damaged portions of Fire Opal, leaving the final stone of the highest quality possible. And these craftsmen refrain from cutting off too much, to prevent compromising the final size of the gemstone.
Another challenge about working with Fire Opal is avoiding further internal fracturing resulting from cutting friction. The vibrations and friction from cutting and grinding will also generate heat that can cause the opal to develop pits on the surface or fracture. Although proper handling can reduce the risk of fracture during this process. Typically, gem cutters use water to keep down the temperatures and mitigate the chance of damage. Finally, the craftsman polish the stone and prepare it for sale as a loose stone or for jewlery mounting.
Today, geologists and gemologists classify opal into two main categories: common and precious. To illustrate, common opal can exist in a wide variety of colors but displays only one primary color with no rainbow overlay. Whereas precious opal displays the fantastic “play of color” for which this gemstone is most famous. Possibilities include the full spectrum of the rainbow. And the spectacular colors observable in precious opal are the result of light refraction, or bending, within the stone.
When white light hits an opal, some of the light changes direction as it enters the denser material. As the light bounces off the silica spheres comprising the opal, light exits the stone at varying angles and displays differing spectrums of visible light. Similarly, a rainbow forms by the refraction of sunlight by water droplets in the sky. In the case of precious opals, the refracting surface is the gemstone, and the rainbow is in the opal!
Opal can form in any subterranean host material with crevices that silica can collect. This means that rocks, wood, and even bones can be suitable for the formation of opals. For instance, if the silica-rich precursor to opal formation finds its way into a bone or tooth that has been partially rotted away, the solution can fill the cavity and solidify as opal. The result is an opal, which can be common or precious, embedded in a fossil. Although, Fire Opal are less common in fossil circumstances. The opal, in this case, replaces the organic matter as it decomposes to form the hole.
It is also possible for the silica solution to fill much smaller holes in porous organic matter, such as a shell or tooth that is still in-tact. In this case, the opal will form throughout the host material, preserving the internal architecture of the fossil remains. Opalized fossils can occur anywhere that is conducive to opal formation, but opalized dinosaur fossils have only been found in Australia. The country recently established the Australian Opal Centre to collect, preserve, and display these truly remarkable and rare specimens.
Written By: Michael Joseph Sabo
Edited By: Adam Azad Kaligi
Forward-Thinking Geologists, Engineers & Contractors!