Onyx

Onyx is a variety of chalcedony quartz characterized by its straight, parallel bands of alternating color. While the popular imagination associates onyx exclusively with solid jet black, the mineral in its natural form actually occurs with bands of black and white, brown and white, or various combinations of these tones. It is distinguished from the closely related agate by the geometry of its banding: agate displays curved, concentric bands while onyx features straight, parallel layers. Both varieties have been prized since antiquity, but onyx holds a special position in the history of carving and engraving as the premier material for cameos and intaglios.

Formation and Geology

Onyx forms through the same fundamental process as all chalcedony—the slow deposition of microcrystalline silica from hydrothermal fluids. The process typically begins in the gas cavities (vesicles) and fractures of volcanic rock, where silica-saturated groundwater circulates and precipitates fine layers of quartz over geological timescales. The characteristic straight banding distinguishes onyx from agate because of the specific geometry of the cavity or fracture in which the silica deposits—flat, planar voids favor straight layers; rounded cavities produce the concentric arching characteristic of agate.

The coloring of individual bands is determined by trace impurities in the silica solution at the moment of deposition. Black layers form when manganese or carbon compounds are incorporated; white layers consist of nearly pure silica with minimal impurities; brown and reddish-brown layers (as in sardonyx) contain iron oxide impurities. The alternation of dark and light layers reflects periodic changes in the chemistry of circulating fluids over thousands to millions of years.

Major commercial sources include Brazil (particularly the Rio Grande do Sul state, which supplies enormous quantities of raw material for the gem and ornamental stone industry), India, Madagascar, Uruguay, and Germany. The United States, Pakistan, and several African nations also produce notable quantities.

Physical Characteristics

Onyx shares the fundamental physical properties of all microcrystalline quartz. Its hardness of 6.5 to 7 on the Mohs scale places it above most building materials and most metals, making it resistant to scratching in everyday use. The lack of cleavage is a significant advantage—unlike many gem minerals that can split along crystallographic planes, onyx fractures conchoidally (in smooth curved surfaces like broken glass), which makes it predictable and safe to carve and wear.

The specific gravity is a narrow 2.65–2.67, consistent with other quartz varieties. The luster ranges from vitreous (glass-like) on freshly fractured surfaces to silky or waxy on polished faces. Translucency varies from nearly opaque to slightly translucent at thin edges, though most gem-quality pieces are fully opaque.

The refractive index (approximately 1.530–1.540) is characteristic of chalcedony. Under ultraviolet light, onyx typically shows little to no fluorescence, though some treated specimens may exhibit unusual responses.

Varieties of Onyx

True Onyx: Technically, pure onyx has alternating black and white straight bands. This form is most valued for cameo carving because the white and black layers allow sculptors to carve relief images in one color against a contrasting background.

Sardonyx: Perhaps the most historically significant variety, sardonyx has bands of sard (reddish-brown, colored by iron oxide) alternating with white bands. It was the most prized carving material in ancient Rome and Greece.

Cornelian Onyx (Carnelian Onyx): Alternating bands of orange-red carnelian and white chalcedony, used extensively in ancient Egyptian and Roman jewelry.

Arabian Onyx: A trade name for the black-and-white banded variety most commonly used in cameos.

Solid Black “Onyx”: The most commercially dominant form today—almost always dyed gray chalcedony or agate, not naturally occurring solid black onyx (which is extremely rare in nature).

Dyed Black Onyx: A 2,000-Year-Old Practice

One of the most important facts about black onyx is that the vast majority of material sold under that name is treated. Natural solid black onyx—chalcedony with uniformly black coloring throughout without the characteristic white bands—is exceedingly rare in nature. Since at least Roman times, craftspeople have dyed gray or light-banded chalcedony and agate to create uniform black material.

The traditional method, still used today, involves soaking the raw stone in a concentrated sugar or honey solution for weeks, followed by immersion in concentrated sulfuric acid. The acid carbonizes the sugar that has permeated the stone’s pores, depositing fine carbon particles throughout the interior and producing a permanent deep black color. Alternatively, treatment in ferrous iron solution followed by heating in sulfuric acid produces a similar result. These treatments are fully accepted by the gemological community and by gem trade standards bodies, provided they are disclosed. The treatment is essentially permanent under normal conditions.

This means that when buying black onyx, the color you see is very likely enhanced, but this is not considered deceptive—it is standard trade practice with a history stretching back over two millennia.

Historical and Cultural Significance

Onyx carries one of the longest documented histories of use of any gemstone. In ancient Mesopotamia and Egypt, banded onyx and sardonyx were worked into beads, amulets, and decorative objects as far back as the third millennium BCE. Egyptian artisans used it for bowls, vases, and ornamental inlays.

The ancient Greeks and Romans prized onyx and sardonyx above almost all other stones for engraved gems—the art of carving images in relief on gemstones. Sardonyx, with its alternating red-brown and white layers, was especially coveted because sculptors could carve a white portrait on a dark reddish background with extraordinary precision and detail. Some of the finest surviving examples of ancient gem engraving are executed in sardonyx. Roman emperors used large sardonyx intaglios as official seals.

The name onyx derives from the ancient Greek word onyx (ὄνυξ), meaning “fingernail” or “claw.” One Greek myth explains this etymology: the goddess Aphrodite was sleeping near Eros, and he clipped her fingernails with an arrowhead while she slept; the clippings fell into the Indus River and transformed into stones (onyx), since the gods’ divine substance cannot be lost.

Onyx is mentioned several times in the Bible, including as one of the twelve stones in the High Priest’s breastplate (Exodus 28:20) and as a material found in the Garden of Eden (Genesis 2:12). In Islamic tradition, it is mentioned in the Hadith as a stone that relieves childbirth pain.

Onyx in the Art Deco Period

The early 20th century witnessed a dramatic revival of interest in black onyx, particularly during the Art Deco movement of the 1920s and 1930s. Art Deco jewelry designers embraced the bold geometric aesthetic of the period, and jet-black onyx paired with white diamonds became the defining combination of the era. Cartier, Van Cleef & Arpels, and other prestigious houses produced extraordinary pieces combining these two stones in geometric patterns of extraordinary precision.

The contrast of black and white—opacity and brilliance—captured the spirit of the age: modernity, clarity, and elegance. Onyx panels also appeared in the architecture of the period: the Chrysler Building in New York and many Art Deco cinemas and hotels featured black onyx wall panels and decorative elements.

Cameo Carving Tradition

The art of cameo carving—relief sculpture in which an image projects above its background—reached its pinnacle using banded onyx and sardonyx. The technique exploits the stone’s layers: the carver removes material from the top layer (usually white) to create the raised image, while the underlying layer (black, brown, or red) provides the contrasting background.

This art form reached its peak in classical antiquity and was revived during the Renaissance, when Italian craftsmen in Rome and Florence produced masterpieces of the form for European nobility. Today, skilled cameo carvers continue to work in Torre del Greco, near Naples, Italy, which has been a center of cameo production since the 18th century.

Comparison with Similar Stones

Agate: Onyx and agate are essentially the same material—both are chalcedony—but differ in banding geometry. Agate has curved, concentric bands; onyx has straight, parallel bands. In practice, the distinction is often overlooked in trade.

Black Tourmaline (Schorl): Natural jet-black tourmaline can superficially resemble solid black onyx but has different physical properties—it is harder (7–7.5), has a slightly different luster, and often shows striated crystal faces.

Jet: An organic material (fossilized wood) that is also deep black and has been used in mourning jewelry. Jet is much softer (2.5–4) and much lighter in weight than onyx.

Black Glass: Manufactured black glass (sometimes called “French jet”) is lighter, has a different fracture, and scratches more easily than onyx.

Buying Tips

When buying black onyx jewelry, remember that nearly all of it is dyed chalcedony—this is fine and fully acceptable, but be cautious of sellers who claim their black onyx is “rare natural” or charge extreme premiums based on supposed rarity. For cameos or carved onyx with natural banding, look for pieces where the carving clearly exploits the natural layer structure; this is a sign of genuine banded onyx rather than dyed solid material.

The quality of cut and polish matters considerably. Well-polished onyx develops a beautiful mirror-like black luster; poorly polished pieces look flat and gray. For rings and bracelets, look for well-executed cabochons without scratches or surface pitting.

Care Guide

Onyx requires straightforward care. Clean with warm water and a mild soap using a soft brush; rinse well and dry completely. Avoid harsh chemical cleaners, which can affect dyed stones. Store onyx separately from harder gemstones—diamond, sapphire, and topaz can scratch the surface if they come into contact. Although onyx is durable for everyday wear, direct sharp impacts against hard surfaces can cause conchoidal fractures, so exercise some care during physical activities. Prolonged exposure to strong acids should be avoided, as they can attack silica surfaces over time.

Metaphysical Properties

In crystal healing traditions, black onyx is considered a stone of protection, strength, and grounding. It is believed to absorb and transmute negative energy, making it a popular choice for protective amulets and talismans. It is associated with the root chakra and is said to promote endurance, perseverance, and self-control. In many cultures, it is worn to ward off the “evil eye” and protect the wearer during times of stress or conflict.