The marketplace for gemstones and decorative materials often uses names that can be confusing, blending geological definitions with trade names. This is especially true when a stone becomes popular for a specific, striking color that may not occur in nature. When consumers encounter the deep, vibrant hues of blue onyx, questions of authenticity naturally arise. To understand the truth behind the striking blue shades, it is necessary to examine the mineral composition of true onyx and the common practices of the gem industry. This exploration will clarify the origin of the color and provide practical knowledge for identifying treated materials.
What Onyx Is and Its Natural Color Palette
Onyx, in its precise gemological definition, is a form of chalcedony, a microcrystalline variety of the mineral quartz (silicon dioxide). It is distinguished from other chalcedony varieties like agate by its straight, parallel banding structure. The traditional color range for true onyx includes black, white, brown, red, and various shades of gray, often seen as alternating parallel bands. A separate material, often marketed commercially as “onyx marble,” is actually banded calcite or limestone. Neither true onyx nor onyx marble naturally occurs in the vivid blue commonly seen in the market.
The Authenticity of Blue Onyx
The rich, deep blue onyx widely available in jewelry and decorative slabs is not a naturally occurring stone color. When the term “Blue Onyx” is applied to material with a saturated, uniform blue hue, it almost always refers to a treated or enhanced stone. The base material, chalcedony, simply does not crystallize with that intense coloration naturally.
Some materials sold under this trade name do have natural blue tones, but they are often not true onyx. For instance, naturally occurring banded blue calcite is sometimes sold under this moniker, but it is geologically distinct and less durable than quartz-based onyx. Natural blue chalcedony also exists, but its color is typically a pale, grayish-blue hue, derived from light scattering or trace mineral inclusions, not the deep saturation of a dyed product.
Techniques Used for Color Enhancement
The vibrant blue color in commercial “blue onyx” is achieved through a permanent and deliberate dyeing process. This treatment exploits the porous nature of the base material, which is usually a pale, inexpensive chalcedony or agate. The microscopic pores and channels within the stone’s structure allow it to absorb chemical solutions. For blue coloration, the stone is typically immersed in a chemical bath containing a blue-producing agent.
Historically, this has involved compounds like potassium ferrocyanide followed by an iron salt solution to precipitate a blue pigment, known as Prussian blue, within the stone’s pores. The stone is soaked in this solution for a prolonged period, sometimes days or weeks, to ensure maximum color penetration. After the soaking, the material is often heated to stabilize the dye and make the color permanent. This process effectively locks the artificial pigment deep within the stone’s structure.
How to Identify a Treated Stone
Consumers can employ several visual and physical checks to determine if a blue onyx specimen has been treated with dye. The most telling indicator is the color’s distribution and saturation; an unnaturally uniform or “electric” blue color is a strong sign of enhancement. Look closely at the surface and any natural fissures or lines present in the stone. Dye tends to concentrate in these porous areas and surface-reaching cracks, appearing as darker, more intense lines of color. This visible concentration of pigment is a clear sign of treatment.
Testing for Surface Dye
A simple test involves gently rubbing a cotton swab moistened with acetone or alcohol on a small, inconspicuous area of the stone. If any blue color transfers to the swab, it indicates a surface-level dye that has not been properly stabilized. Additionally, true chalcedony onyx should feel cool to the touch and possess a hardness of 6.5 to 7 on the Mohs scale.