Onyx is a form of chalcedony, a microcrystalline variety of quartz, historically valued for its distinct parallel layers of color. The stone’s name comes from the Greek word meaning “claw” or “fingernail.” For millennia, onyx has been a favored material in jewelry and decorative arts, lending itself well to carvings like cameos and intaglios. Understanding the origin of black onyx requires looking at the geological environments that create it and the commercial processes that give it its familiar, uniform color.
Geological Identity and Formation
Onyx belongs to the mineral group chalcedony, a cryptocrystalline form of silica (silicon dioxide). What distinguishes onyx from agate is the arrangement of its bands, which are consistently straight and parallel, rather than curved. The stone forms through precipitation, where silica-rich fluids deposit layers within cavities and fissures in the host rock.
Formation typically begins in regions with volcanic activity, where gas bubbles in cooling lava create voids. Silica-laden groundwater seeps into these open spaces, depositing microscopic quartz crystals in rhythmic layers. The characteristic banding results from variations in the water’s chemical composition, which changes the type or amount of trace mineral impurities deposited.
Iron oxides, manganese, and other elements determine the color of each layer, ranging from white to red, brown, or the rare natural black. The resulting mineral is durable, possessing a hardness of 6.5 to 7 on the Mohs scale. This requirement for silica-rich solutions and pre-existing rock cavities dictates that onyx deposits are found in specific, often volcanically-influenced, geological settings worldwide.
Primary Global Sources of Raw Onyx Material
The raw chalcedony material processed into black onyx is sourced from numerous locations meeting the geological conditions for its formation. Brazil is one of the world’s leading producers of raw agate and chalcedony, forming the basis for much of the commercial onyx supply. Brazilian deposits, particularly in the northeastern state of Piauí, often yield material with characteristic reddish-brown, white, and beige banding.
Uruguay is another globally recognized source, known for producing high-quality chalcedony used in the gem trade. India also has significant deposits in the state of Madhya Pradesh, yielding stones with clearly defined black and white bands. These regions provide the raw, layered stone that is the starting point for the black onyx trade.
In North America, Mexico is known for substantial reserves, with mining operations in states like Sonora and Oaxaca. Mexican chalcedony is varied, often including translucent to opaque varieties with reddish hues. Other contributors include Argentina, Pakistan, and Turkey, which possess the necessary volcanic or silica-rich geology to produce high volumes of raw banded stone.
Distinguishing Natural Black Onyx from Commercial Treatments
The black onyx most people encounter in jewelry is rarely in its naturally occurring state. True, completely natural solid black chalcedony is exceptionally rare, and even naturally black-and-white banded onyx is uncommon. The vast majority of the uniform, jet-black stone sold commercially is agate or grey chalcedony that has been permanently treated to achieve its consistent coloration.
This treatment relies on the porous nature of chalcedony, which allows it to absorb liquids. The most common method involves soaking the raw, often grey or light-colored, stone in a sugar solution, followed by immersion in sulfuric acid. The acid reacts with the absorbed sugar, causing carbon to precipitate as a fine black pigment within the stone’s microscopic pores, rendering the stone a deep, stable black.
This carbonization technique has been used since Roman times and is considered a standard enhancement within the gemological trade. The resulting material is still considered genuine onyx because it is a natural mineral with the same chemical composition and hardness as untreated stone. The commercial term “black onyx” now refers to any solid black chalcedony, regardless of whether its color is natural or the result of this dyeing process.