Almost all blue topaz available on the market has been treated with radiation followed by heating. Colorless topaz is first irradiated, turning it brown. It is then heated to produce different shades of blue: dark blue-gray (London blue), bright and saturated blue (Swiss blue), or pale blue like seawater (Sky blue). Natural blue topaz is often pale, so this treatment, which is very stable and accepted, enhances its color.
There are multiple methods for irradiating topaz.
High-Energy Electron Irradiation
Uses a stream of electrons to induce color, and is the most common method for coloring Swiss blue and sky blue topaz. These electrons are accelerated by equipment such as linear accelerators. This treatment can cause intense surface heat, leading to issues like cracking or internal electrical discharge. This method is safe and doesn't pose a risk to humans.
Neutron Irradiation
A stream of neutrons penetrates deeply into stones, ensuring uniform coloring without causing cracks. However, this treatment can make topaz radioactive, requiring a cooling period. Topaz treated with neutrons can continue to emit radiation for years, so it must be thoroughly checked before it can be sold. The Nuclear Regulatory Commission (NRC) mandates that irradiated gems be identified as such and that enough time passes for short-lived activation products to decay before they can be distributed in the U.S. This ensures the safety and compliance of irradiated blue topaz in the market.
Gamma Irradiation
Gamma irradiation can change the color of topaz from colorless to blue, yellow, or brown. Heat treatment can then be used to eliminate the unstable yellow color, resulting in sky blue topaz. The process involves exposing the topaz to gamma rays emitted by radioactive isotopes of cobalt or cesium, which are sealed in stainless steel rods for safety. The irradiation process also generates a lot of heat.Ā
Nuclear Reactor Irradiation
Nuclear reactor irradiation can turn topaz green
These treatments are performed by radiation experts and specialized companies that have refined their techniques over decades. The process starts by turning colorless topazes into yellow, green, or brown shades. Heating them then achieves the desired blue hues. This process, known since the 1950s and popular since the 1970s, has greatly increased the availability and popularity of blue topaz in the market.
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