Onyx, Obsidian and Jet: Black Gemstone Differences

Three Black Gemstones, Three Very Different Characters

Hold a black onyx cabochon, a lump of polished obsidian and a Victorian jet bead in your palm and they look like close cousins. Wear them for a week and the family resemblance fades fast. One shrugs off knocks, one chips if you look at it sharply, and one weighs so little you forget it is there at all.

Black stones are a mainstay of nautical jewellery — set in silver anchors, hung beneath compass pendants, threaded onto rope bracelets. Knowing which black you are buying means knowing whether your piece will survive a summer of salty hands and clinking against a boat rail. Here is how onyx, obsidian and jet actually differ, and which settings suit each.

Where Each Stone Comes From

Onyx is a form of chalcedony, which puts it in the quartz family. It forms when silica-rich water seeps into cavities in rock and hardens in layers. Much of the deep black onyx on the market is banded agate that has been dyed and heat-treated — a long-established practice, not a swindle, but worth asking about when the colour matters to you. Brazil, India, Madagascar and Uruguay are the usual sources.

Obsidian is glass, not mineral. It forms when lava cools so quickly that crystals never get the chance to grow. It is natural volcanic glass with a sharp, glassy edge — ancient peoples used it for blades. Mexico, Iceland, Italy, Japan and the western United States all produce it, in plain black as well as snowflake, rainbow and mahogany varieties.

Jet is the odd one out: it is fossilised wood, a compact form of lignite, and therefore organic rather than mineral. Whitby in North Yorkshire has been the most famous source since the Victorian era, when jet mourning jewellery became a national fashion after Prince Albert's death. Spain, France and Germany also yield jet, but Whitby material carries the story.

Hardness and Everyday Durability

On the Mohs scale, onyx sits at 6.5 to 7, which is genuinely tough for a decorative stone. It resists scratching from keys, coins and car doors, which is why it has been the stone of choice for signet rings for centuries.

Obsidian measures around 5 to 5.5. That is respectable, but it has no crystal structure to absorb shock. It fractures in smooth curved shells, so a knock on a hard edge can take a chip out of it, and the resulting edge is razor-sharp.

Jet is soft, at 2.5 to 4. It scratches against almost everything, including itself, and it is brittle. It also weighs almost nothing — a large jet brooch can be worn on a light blouse without dragging, which is exactly why Victorian jewellers loved it.

Lustre, Feel and Light

• Onyx takes a high, glassy polish and looks almost liquid in a well-cut cabochon. It is cold and dense in the hand, and the black is deep and even.
• Obsidian can be glassier still, sometimes with a silvery sheen under the surface — snowflake obsidian shows grey crystalline patches, while rainbow obsidian flashes colour in strong light. It feels slippery and smooth, and heavier than you expect for a glass.
• Jet is the quiet one. Polished jet has a soft satin sheen rather than a mirror shine, and it feels warm to the touch within seconds, a trait no mineral stone shares. Rub it briskly and it will pick up static and attract dust.

Matching Stone to Setting

For rings worn daily, onyx is the sensible choice. It handles grooves, engraving and the knocks of a working hand, and it takes fine detail beautifully in cameos and signets. Pair it with sterling silver or stainless steel for a crisp coastal look.

Obsidian belongs in pendants, brooches, earrings and statement cuffs, where it is unlikely to meet hard edges. If you do want it in a ring, choose a bezel or a well-protected dome rather than a faceted stone with exposed corners, and treat it as an occasional piece.

Jet suits exactly what it has always done best: carved brooches, beads, pendants, cufflinks and mourning-style pieces. Its lightness makes generous, sculptural designs feel effortless. Avoid it in rings entirely, and think twice before putting it on a charm bracelet that will swing against metal all day. On a nautical piece, jet pairs beautifully with oxidised silver — the matte black against grey metal looks understated rather than flashy.

Cleaning and Long-Term Care

Onyx is easy: warm water, a drop of mild washing-up liquid and a soft brush. Skip ultrasonic cleaners on dyed stones and keep them away from prolonged heat, which can dull the treatment.

Obsidian needs a gentler hand. Wipe it with a damp cloth, dry it, and store it in a pouch or lined box so it does not clink against harder stones. Never use steam or ultrasonic cleaning, and check periodically for small chips along edges.

Jet is the one to fuss over. Keep it away from perfume, hairspray, sun cream and alcohol-based cleaners, all of which will strip its polish. Clean it with a dry, soft cloth rather than water, store it separately, and do not leave it on a sunny windowsill.

Buy the stone that matches the life your jewellery will lead, and any of these three blacks will serve you well for decades of coastal wear.