Topaz is a durable aluminum silicate mineral found worldwide. While it occurs naturally in various colors, including colorless, yellow, and brown, blue is one of the most popular hues in the jewelry market. Naturally blue topaz exists, but virtually all commercially available blue topaz has been subjected to treatment.
The Rare Case of Natural Blue Topaz
Naturally colored blue topaz is exceptionally rare, making up a tiny fraction of the global supply. When it occurs, the color is typically a very light, desaturated sky blue, often mistaken for other light blue gemstones like aquamarine. The subtle blue hue results from minute trace elements interacting with natural defects within the crystal lattice.
These natural radiation processes create “color centers” that selectively absorb light, manifesting as a delicate blue color. Because of its scarcity and subdued color, natural blue topaz rarely meets the market demand for saturated, vivid blue stones.
The Treatment Process for Vivid Blue Hues
The intense, saturated blue shades that dominate the market are achieved through a controlled, two-step modification process applied to colorless or light-colored topaz. The starting material is usually clear or pale yellow topaz, which is readily available. The first step involves exposure to ionizing radiation to fundamentally alter the internal crystal structure and create specific “color centers” within the gemstone’s lattice.
Less intense methods use electron bombardment, typically yielding lighter blue shades. For deeper, more intense colors, the topaz is subjected to neutron bombardment, a more powerful process requiring specialized reactor facilities. The duration and intensity of the irradiation determine the potential depth of the resulting blue color.
Following irradiation, the second step involves controlled heating (often ranging from 300°C to 500°C). This high-temperature process serves a dual purpose. Heating stabilizes the newly formed color centers, ensuring the blue hue remains permanent and will not fade. The heat treatment also helps eliminate unwanted secondary colors, such as brown or greenish tints, resulting in purer blue saturation. This combined irradiation and heating process is universally accepted and transforms topaz into marketable blue gemstones.
Trade Names and Color Intensity
To differentiate the varying shades produced by treatment, the jewelry trade uses specific, non-geographical names to describe color intensity. These terms are marketing labels corresponding to the saturation level achieved through irradiation and heating.
Sky Blue
The lightest shade is Sky Blue, characterized by a pale, airy blue that closely resembles the faint color of natural blue topaz. This lighter color usually results from the less intense electron beam irradiation.
Swiss Blue
A step up in saturation is Swiss Blue, which presents a medium, vibrant, and highly electric blue hue. The vividness of Swiss Blue requires a more significant dose of radiation than Sky Blue, pushing the color centers to a greater concentration. This shade often represents a balance between intensity and brightness, making it a popular choice.
London Blue
The deepest and most intense shade is London Blue, characterized by a dark, inky blue with a noticeable grayish or steel undertone. Achieving this rich saturation requires the most powerful treatment, typically involving the expensive and prolonged neutron bombardment phase. Because of the complexity and extended timeline, London Blue stones are generally the highest priced.
Identifying Blue Topaz Versus Other Gemstones
While the color of treated blue topaz is distinctive, consumers should be aware of common simulants used as substitutes. Treated quartz, sometimes marketed as mystic topaz, lacks the internal structure and density of true topaz. Synthetic spinel is another common look-alike manufactured to mimic the vibrant blue hues.
Topaz possesses a distinct set of physical properties that help distinguish it from these alternatives. Topaz ranks an 8 on the Mohs scale of mineral hardness, significantly harder than quartz (7) and offering better resistance to scratching. Topaz has a relatively high density, meaning a stone will feel noticeably heavier than a similarly sized piece of quartz or glass. Definitive identification requires professional gemological testing, which measures the stone’s precise refractive index.