Ouro Verde Quartz: History and Treatment
Ouro Verde quartz, also known as lemon quartz, is a popular treated stone. Learn how experiments with other gems led to the development of this treatment.
6 Minute Read
What is Ouro Verde Quartz?
Traded in Brazil under the name Ouro Verde, Portuguese for "green gold," this quartz material has a light yellow/green color similar to chrysoberyl. (Hence, its other name, "lemon quartz"). This material isn't "nuked," which implies nuclear irradiation. Instead, it's irradiated with cobalt-60 gamma rays at a very low dosage, which dissipate as soon as they go in. Next, the gem is heated in a household toaster oven in several steps.
I'm going to discuss how I discovered the Ouro Verde process and also answer this question: why did it take so long for the gemstone industry to discover radiation treatments for quartz material other than smoky quartz?
You must understand that the art of gemstone radiation treatment is relatively new. By the 1960s, diamonds had been successfully — but primitively — irradiated. What started the ball rolling for quartz and other gemstones was the discovery of a treatment to create blue topaz. This happened mainly because the profit incentive was there.
Blue Topaz Treatments Led to Experimentation
The first topaz treatment was a simple procedure with cobalt-60 that produced, in the right materials, only a very light blue stone. (This irradiation process can actually occur in nature, too, but very rarely and slowly). Prior to this discovery, natural light blue topaz fetched $300 a carat in Brazil. On the other hand, white or colorless topaz occurred so abundantly most considered it almost worthless. You could buy ton-lots of giant white topaz from the Marambaia mine in Brazil — the best material for treating — for $5 to $10 a kilo. Flawless stones weighed up to 20-30 kilos.
Naturally, this type of incentive pushed investment in R&D. Soon, discoveries showed that bleaching prior to treatment, different dosing rates, and different types of radiation could improve results. As a result, we now have an assortment of topazes never encountered in nature, including iridescent greens and super blues.
The topaz came out of the irradiation treatment an opaque black color. A low-level heating followed, at 350-500° F for one to two hours. At the end, you had all blue stones, with the exception of a few pieces that stayed opaque black. These stones were actually quartz. Many folks just trashed them.
I sold these black quartz pieces by the pound for use as black onyx. Since the irradiated black color went all the way through, they were exceptionally well-suited for this purpose. (Typically, black onyx is created by soaking a gray agate in honey or sugar, then placing it in heated acid. This turns the color black, but only to a depth of a few millimeters).
Manufactured mainly in Germany and sold in slabs, commercial black onyx fetched, at best, $100-$200 a kilo at the time. The preferred gray agate for this treatment, from Rio Grande do Sul, Brazil, still blankets the ground there and sells for only a little over the freight costs.
So, I had opaque black quartz that would only bring $100-200 a kilo. My irradiation costs could get up to $1,000 a kilo at the time. Taking this material to the next higher heating temperature required a closed crucible, since you need a casing oven to surpass 500° F. Trying to salvage an apparent loss, I experimented with various heating combinations, but they all involved higher temperatures that needed closed-door equipment. Previously, I had used a toaster oven with a glass door so I could watch the results and know when to stop. The best I achieved with a casting oven was smoky quartz. Unfortunately, I usually just managed to turn these pieces back into clear, white quartz, which had a lower value. (Actually, the smoky quartz wasn't much better value-wise).
I eventually learned that it only takes 0.5-1 megarad of cobalt-60 to make pretty much all quartz turn blackish. Still, the value of the treated quartz was only $5-$10 a kilo. So, again, I actually increased my loss with the extra work and electric bills.
Experiments with Beryl
As the blue topaz market waned, I began experimenting with irradiating white beryl. I discovered the effects differed considerably, depending on the temperatures of the subsequent heating. At this time, I was living in Minas Gerais, Brazil with 15 workers cobbing the beryl flawless at the source.
I hoped that irradiated beryl would become the next "blue topaz rage." However, beryl presented several problems. The availability of white beryl is sporadic. Your treatment results also vary within a given mine, even from stone to stone. To treat beryl at that "blue topaz" commercial level, you would need a continuous supply of economically feasible quantities of rough with uniform quality. Most importantly, you would also need to know the required radiation dosage and its effects on the majority of the treated beryl.
To achieve top colors, most beryl pieces need dosages of 5,000 megarads and a high dosage rate. A linear accelerator with electron irradiation can best achieve this, but it's prohibitively expensive. Furthermore, since beryl can have a reactive cesium content, you might end up with a product that needs to cool for 100 years. You could achieve the same results, without the drawback of residual radiation for long periods, by repeated dosages over one to two years. Irradiation results are cumulative.
As you can surmise, overcoming these obstacles in order to jumpstart a mass commercial fad would be pretty tough.
A Centuries-Old Practice Leads to a Breakthrough
Curiously, after my experiments with beryl irradiation, I once again had those stubborn black quartz pieces. They were mixed in with my beryl pieces. However, this time I was lucky enough to observe Brazilian lapidaries heating aquamarines and tourmalines with just a glass test tube over a Bunsen burner.
This art has been practiced for centuries. Since you could only fit so many stones in a little test tube, I'd never considered it. You could never supply the world with a product line this way, but at least I could see through the glass and get an idea of what would happen to the beryl pieces from different mines.
It worked! The next step was to determine the exact dosages for the exact beryl types from specific locations, along with the proper heating temperatures. This seemed like a life's work. However, here was my opportunity to see the exact progression of the irradiated quartz through heating as well.
Discovering Multiple Colors for Treated Quartz
Previously, I'd observed what I considered the usual color progression of quartz when amethyst is accidentally heated and creates citrine. If you open the oven door in the middle of the process, you might think you overheated the stones to white. The amethyst first loses its purple color and goes clear, then gets citrine colors.
So, I wondered if the irradiated black quartz passed through a middle color during the heating process, too. (As my luck would have it, all clear quartz doesn't take on other colors when irradiated, which further complicated and delayed this discovery).
The first pieces I heated in a test tube went to a glowing canary, almost electric, yellow. Excited, I heated a couple of kilos of irradiated quartz that I had kept from the blue topaz days. I had picked up these pieces during my travels, so they came from locations scattered all over the world. Unfortunately, now I didn't know their origins. However, much to my delight, I got almost every color of the rainbow. I got lime greens, tourmaline greens, every shade of orange, and even red.
In the following year, as more of the world's quartz locations were tested, we received some fantastic new color varieties of quartz. Most importantly, quartz occurs in abundance, and I've observed very little deviation in color from any given location. It's still inexpensive and the necessary radiation dosages are low and in a practical, accessible range. In other words, we had all the ingredients for a commercial success!
The late Mark Liccini began his career as a stone carver in the 1970s and worked for many decades as a renowned lapidary. Mark mastered the art of gemology, gaining degrees from the Gemmological Association of Great Britain and the Columbia School of Gemology.
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