Treating Quartz to Create Color

by Mark Liccini

Some, but not all, clear quartz  can be treated to create numerous colors using irradiation and heat treatments. When treating quartz, it is possible to achieve a range of colors, depending on where the quartz came from and how you treat it. Here is a list of some of the colors achieved using a random lot of quartz from unidentified locations. Canary, golden, straw yellows, all shades of orange from light to dark, all shade of purple from light to dark, various shades of green from light to neon, caramel (a brownish orange or reddish brown), lime green, and red.

The process varies depending on the specific location, but the basic formula is to irradiate the stones at 60 megarads of cobalt 60, X-rays, or with electrons. The material will return from the lab an opaque black or brown. Then the stones are heated in a household toaster oven to 650 degrees F. The maximum temperature listed on a household oven is 550 F, but if left on at the maximum setting for long periods, the temperature will rise above the rated settings. A glass door laboratory furnace would be a better tool if the economics justify it. The ability to see the heating action is absolutely necessary as each individual piece will turn from black to color at a different time.

This basic process can be modified to achieve different effects depending on the location where the quartz was mined. In some instances, one might only achieve color by first destroying the existing color center. This requires heating the crystal until it is clear or white, which may require temperatures up to 550 degrees F. If you should exceed this temperature, you run the risk of creating an opaque, opalized state in which the stone turns to a milky consistency resembling a common opal. If heating to 550 degrees F does not give you a color you desire, you may want to increase the dosage of irradiation or subject the work piece to a different type of irradiation. For example, a quartz that goes to a straw yellow color on cobalt 60, might turn to canary on exposure to electrons. Again, the color you might achieve is dependent on the origin of the material. Some results and individual formulas are listed below.

Smoky Quartz

All quartz can be irradiated to manufacture the morion variety of smoky quartz. In addition, all quartz will turn to a blackish or brownish color on exposure to .5 to 1.5 megarads of cobalt 60, gamma; X-rays; or electrons; however, they will not be completely uniform. If that is your goal you must first irradiate, then separate the lighter ones for reirradition at a higher dosage. Next, separate those that are the optimum color. You can heat those that come back an opaque blackin a household toaster oven (*see heating instruction for furnace operations) until you reach the desired shade. Except for this post irradiation heating step, there is little heat or static electricity in the lab process, so included material can be used. The maximum temperatures the quartz will be exposed to during the irradiation process is 400 F.

Arkansas Quartz (USA)

Tested various areas of this deposit and none took color, all just treated to smoky quartz.

Milky (Silk) Quartz

Tested numerous locations from Brazil, USA, and worldwide. It appears all hazy quartz or quartz containing a silk-like inclusion, no matter what intensity, will treat. This would include your natural rose quartz. The common results were various yellows, an occasional orange cast, or an occasional greenish cast.

In some cases the silk can be cooked out in a pre-irradiation heating. In other cases it can dissolve in the irradiation process and you can even observe the lessening of the silk in the final heating step. Some silk will not dissolve during this heating process and it is also possible that the crystal may shatter during heating, so use caution when performing this step. This material seemed to reach its maximum color saturation at a dosage of 60 megarads. Again, with different shades of color on different types of irradiation.

You can control the color somewhat in the final heating process. After you have irradiated the stones, spread them out evenly on the rack in your toaster oven or a perforated baking sheet. You want to allow air to circulate freely around the stones, so do not place them on a regular cookie sheet. Set your toaster oven to maximum temperature and wait patiently. By continuously observing through the glass door you will see the stones change from an opaque black. Keep in mind that each stone may change at a different time. Larger crystals require more heating time, so it is most efficient to size grade and heat all one size together. If you observe, like most ovens, that your temperature is hotter front to back, you might make the last row larger crystals, so you can observe them better. While they are still heating, you can take out the faster turning smaller crystals in the front row.

You can expect to wait anywhere from about two to eight hours for the crystals to turn starting from a cold oven. The transition can go from opaque black to slightly transparent brownish black, to greenish yellow, to canary yellow. This material changes slowly so you have plenty of time to remove them at the desired color. This is where the glass door comes in handy. You can observe the color change without having to continuously open and close the oven, which would make this process last even longer. Once you are happy with the color of a stone, simply remove the stone with metal tongs and set it on a metal cooling tray. If you leave the stones in at maximum temperature for very long periods, they can fade to light yellow and eventually revert to white.

This is quite a hands-on heating process and the opening and closing of the oven door should be kept to a minimum. Thermal shock will occur if there is too much cool air entering the oven chamber, but it cannot be completely avoided, so expect some breakage.

You can reduce your breakage by waiting until a good number have turned to a desired shade. Once this has occurred, shut off your oven and allow it to cool to room temperature, by cracking the door slightly. If you do not vent the oven, the temperature will continue to rise and you may overheat your stones. Luckily, if you do overheat, all is not lost. Simply reirradiate and begin the process all over.

Caramel Quartz

If you treat quartz from various locations you are likely to encounter this type. I found three mines in the Arasuai area of the state of Minas Gerias, Brazil that produce caramel quartz. None of the stones showed any identifying inclusions and I do not believe they occur well formed. They were presented to me in broken crystals or in cobbed form. One location produced only crystals up to 30 gram and the other two locations produced crystals of 1 to 2 kilos.

These stones were irradiated using the process described under Milky Quartz and heated in the same manner with a different color transition. They go from opaque black to slightly transparent brownish to reddish or orangish brown. On continued heating, some eventually went to a pure orange. These stones required longer heating times. Some crystals, however, stayed brownish orange even after two days heating. I would expect that they might go pure orange at a higher temperature than I was able to achieve with just a toaster oven.

From one mine there was a transition to pure orange. It went from black to a brownish orange and then into a golden.

Neon Green Quartz

There are two adjacent mines just outside of the village of Itacambira, in the state of Minas Gerias, Brazil, that produce colors varying from golden and neon green (Limon) to bicolors and tricolors of golden and black or brownish, in addition to green and black or brownish. In both these mines the crystals are found very clean to flawless, with a maximum size to about 100 grams. They are elongated crystals with an unusual 10-sided termination. These are irradiated using the same procedure, but that is where the similarity ends. The heating process is quite involved and difficult. The crystals return from the lab opaque black. Then it is best to heat them, starting with a lower temperature of 350 degrees F. This will allow you the time to take out the maximum amount of the green ones, which are most valuable. The crystals go from a black, to a slight transparency in the center only. Just the center will lighten and turn green leaving the two ends still opaque black or slightly transparent brown or black.

Unlike milky quartz, the color of neon green quartz lasts from a matter of seconds to a maximum of a minute or two, so you must be quick removing them from the heat as they change. If you delay, the center will turn to a golden yellow color. At that point you will have a tricolor stone, with a green center and blackish ends. If you take too long to remove them, you will have a tricolor with a golden yellow center.

To create a product line that is just green you would, at the first removal step, saw or cob away the two ends, keeping them separate. Then you put all the sawed sections of crystals, from the end away from the termination, back in the oven as these can also go green. Again, you best heat them to 350 F so the change from blackish will be more gradual. If you heat them at a higher temperature, like 550 degrees F, it is more difficult to remove the green ones before they change to golden.

Once you have finished processing the ends away from the termination, you can then process the terminated end. All have some slight inclusions right on the tip of the termination. You can saw it off or knock it off, whichever is easier. When heated, these go mainly golden, although it is possible to have a small portion of green stones. It is impossible to catch the green and golden together because the green comes out at a lower temperature. Right at the termination you will not get any green first, it goes straight to golden. If you overheat at any step, simply reirradiate and repeat the process.

Red Quartz

As noted above, all milky or rose quartz on irradiation will treat to various shades of yellow, from golden to canary. There is very little difference between rose quartz and clear quartz except the color in rose quartz is attributed to submicroscopic inclusions of dumortierite. Dumortierite is a borosilicate mineral that is usually blue or pink. On analysis you may also find the presence of titanium.

There is one location in which rose quartz crystallizes. This deposit is just outside of the small city of Governador Valadares in Minas Gerias, Brazil, in an area called Ganga Rosa. It has been reported to me that there are deposits of well formed crystals in Maine, USA, but to the best of my knowledge there is only one location in which rose quartz forms in terminated crystals — Ganga Rosa.

The crystals there can occur anywhere from colorless, to a hint of pink, to a fine rose color. For the purpose of coloring by irradiation, it does not seem to matter if you begin with well saturated pieces or clear. The process of irradiating quartz to color, as described above, is simply to dose the material to 60 megarads. The subsequent heat treatment can vary depending on the location of the material. So far in my testing shows, quartz achieves its full saturation at a dose of around 60 megarads.

It serves no purpose to give additional higher dosage, except with this very unusual occurrence of Ganga Rosa. Unlike other quartz, which returns from the lab after irradiation an opaque black and needs heating, Ganga Rosa comes back after irradiation to 60 megarads a strawberry red, with medium to light intensity. There is no subsequent heating necessary. The more dosage you give the material the darker the color. My experiments were limited to gamma, using cobalt 60 and X-rays in the form of residual irradiation. It was in the X-ray testing that it was noted the material continues to darken. Crystals were placed just outside of the direct beam cylinder at a linear accelerator laboratory, while the material inside the cylinder was given dosage at 24 MEV of 5,000-10,000 megarads electrons.

We could not gage exactly what dosage was administered to the exterior area containing quartz, but certainly several thousand megarads at an accelerated dosage rate. As explained by the laboratory, this residual irradiation is X-ray (Gamma) only. It is, without a doubt, that given higher dosage, intensity of color can be achieved as dark as ruby. To the experimenter it should be noted there is likely no way to overdose this material, as all irradiated materials can be heated back in stages on low temperature treatments.

Now, another interesting effect can be achieved. I observed on pieces with the mineral calcite, or more likely a calcite pseudomorph, attached to quartz, the attached mineral goes black on irradiation as you would assume a quartz and not a calcite to do. This can create an interesting bicolor effect in some of the pieces.

To the lapidary this should be exciting news. Ganga Rosa is still producing these crystals today, and over the years has produced hundreds of kilos to tonnage. Although it is not a great production, this mine is reported to produce about 200 kilos per month, of which faceting quality may be 10-20 kilo per month. On advent of this paper I expect the market prices to change, but for the moment the only value placed on them is in the collection market. Well formed pieces of good rose color can be quite pricey, but those near colorless or damaged are almost valueless.

A trip to Govenador Valadares would find at this writing, the material readily available sold by Garimpeiros in the Rua, small miners and agents of miners in the gemstone trading street of Rua Picina, Centro. You will find that they are completely unaware of the treatment potential. The less adventurous individuals can simply visit your local Gem and Mineral Show. There you will find dealers displaying this very same quartz, also uninformed that it can be treated to red.

Mark Liccini
Gemstone Rough Dealers since 1970

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