How to tell onyx and obsidian apart

Onyx and obsidian can look remarkably similar in a polished pendant, cabochon or bead. Both are commonly sold in black, both take a smooth shine, and both may be described with broad trade names that make identification less clear than it should be.

The key difference is geological. Onyx is a banded variety of chalcedony, made from microcrystalline quartz. Obsidian is volcanic glass, formed when silica-rich lava cools so quickly that crystals have little time to develop. Their appearance, hardness and natural patterns reflect these different origins.

For Australian buyers, the distinction matters when comparing a market-stall bargain with a jeweller’s labelled piece. A stone sold in Sydney, Melbourne or Brisbane should be represented accurately under the Australian Consumer Law, especially if colour, treatment or gemstone identity affects its value.

Visual inspection is useful, but no single home test proves identity. Surface finish, lighting, inclusions and imitation materials can all cause confusion. A careful comparison, followed by professional testing when the purchase is valuable, gives the most reliable result.

Start with colour and banding

Commercial black onyx is often evenly coloured, although genuine onyx technically has layers or bands produced by successive mineral deposits. These bands may be subtle, parallel and straight, appearing in black, white, brown, grey or reddish tones. In jewellery, the stone may be cut to hide the layers or show only a dark face.

Obsidian usually has a glassier appearance and can show flowing colour zones rather than neat parallel bands. Rainbow obsidian may display curved iridescent patches, while snowflake obsidian contains pale, flower-like spherulites. Mahogany obsidian has reddish-brown areas, and some pieces show a smoky, translucent edge.

Hold the item under a strong but diffuse light, such as daylight near a window. A very thin edge may allow light through in obsidian, while black chalcedony generally appears more waxy and opaque. This is an observation aid rather than a final test, since thin cuts, coatings and lighting conditions can change the result.

Compare lustre, texture and hardness

Polished obsidian tends to have a vitreous, glass-like lustre. Its surface can look sharply reflective, especially on a freshly polished cabochon. Onyx typically has a softer, waxier sheen, although high-quality lapidary polishing can make it quite bright.

On the Mohs scale, quartz-based onyx is around 6.5 to 7, while obsidian is usually about 5 to 5.5. That makes onyx more resistant to everyday scratching. However, deliberately scratching a finished jewel is poor practice: it can damage the setting, leave a visible mark or reduce resale appeal.

A jeweller or gemmologist can examine refractive behaviour, inclusions and surface features with magnification. For an heirloom, a custom ring or a stone bought as an investment, this is safer than relying on a pocket tool or a social-media identification chart. A sapphire ring may involve entirely different symbolism and valuation considerations, as explained in this guide to the meaning behind a sapphire ring.

Use simple checks without damaging the stone

Several clues can narrow the possibilities before professional assessment. Look through a loupe for rounded bubbles or flow lines, which can support a glassy volcanic origin. Onyx may reveal fine parallel bands, though dyed or reconstructed material can obscure its natural structure.

Be cautious with sellers who use “black onyx” for any dark stone. Dyed chalcedony, glass, resin and other simulants appear in inexpensive beads and fashion jewellery. In Australia, a description that creates a misleading impression about composition or quality may raise concerns under the Australian Consumer Law, so retain the receipt and product description.

Useful observations include:

• Onyx: waxier lustre, fine bands and greater scratch resistance
• Obsidian: glassy lustre, curved flow patterns and occasional translucency
• Dyed material: colour concentrated in cracks, pores or drill holes
• Glass imitation: possible round bubbles and an unusually uniform appearance

Consider treatments and buying conditions

Black onyx is frequently dyed to create an even, intense colour. Treatment is not automatically a defect, but it should be disclosed where it affects the nature or care of the item. Obsidian is generally sold in its natural colour, although polishing, carving and coating can alter its appearance.

Ask whether the stone is natural, dyed, assembled or synthetic, and request the information in writing for a significant purchase. Australian jewellery businesses do not generally operate under a universal compulsory gemstone hallmarking system equivalent to some overseas schemes, so a stamp inside a ring usually refers to metal fineness rather than proving the identity of the stone.

Prices also vary with cut, workmanship and provenance. A polished obsidian pendant from a specialist lapidary may cost more than a mass-produced “onyx” bead strand, while rare patterned obsidian can command a premium. Local gem and mineral shows, weekend markets and established jewellers each have different levels of documentation, so compare descriptions rather than judging by price alone.

Care for black gemstones in everyday wear

Onyx is hard enough for regular jewellery but can chip at thin edges or corners. Obsidian is more vulnerable to scratching and edge damage because it is softer and breaks with a glass-like fracture. Remove either stone before gardening, moving furniture, contact sport or hands-on work.

Clean both with lukewarm water, a small amount of mild soap and a soft cloth. Avoid ultrasonic cleaners, steam and harsh chemicals unless a qualified jeweller confirms that the stone and setting can tolerate them. Dyed onyx may be especially sensitive to prolonged soaking, heat or solvents.

Care reminders:

• Store each piece separately in a soft pouch
• Keep obsidian away from harder gemstones
• Wipe off sunscreen, perfume and sweat after wear
• Check prongs and clasps during routine cleaning

For symbolism-led gifts, colour can matter as much as mineral identity. A warm golden or blue topaz may be chosen for personal associations, and this guide to topaz colour variations shows why trade descriptions should be separated from verified gemological facts.

When the distinction affects price, insurance or a sentimental purchase, obtain an independent assessment. A qualified Australian gemmologist can identify the material, note treatments and provide a clearer basis for care and valuation.