The Difference Between Onyx and Obsidian Gemstones

Black, glossy and undeniably eye-catching, onyx and obsidian sit side by side in many a jewellery box — and from arm's length they can be almost impossible to tell apart. Yet these two dark stones are about as different as two gems can be. One is a variety of quartz; the other is cooled lava. Knowing which is which matters when you are choosing a ring you intend to wear every single day, or deciding how best to look after a treasured pendant.

Here is a practical guide to both, and to the small clues that give each one away.

Why the Confusion Happens

Both stones are opaque, both take a high, glassy polish, and both are commonly cut into cabochons, beads, signet-style plaques and carved cameos. The naming tradition does not help. The word onyx originally described any stone with parallel layers of colour, and it has been borrowed freely over the centuries for anything dark and polished. Much of the "black onyx" sold today began life as pale grey chalcedony before being dyed, so a shopper comparing dyed onyx with natural black volcanic glass sees two very similar-looking objects. The differences only become obvious once you know what each stone is made of.

What Onyx Really Is

Onyx is a variety of chalcedony, which itself is microcrystalline quartz — silicon dioxide, the same chemistry as amethyst, citrine and rock crystal. The difference is the crystal size: the grains in chalcedony are so tiny that they cannot be seen with the naked eye, which gives the stone its dense, waxy, tough character.

• Hardness: around 6.5 to 7 on the Mohs scale, so it resists scratches well and suits everyday rings.
• Structure: natural onyx forms in flat, parallel bands. Sardonyx shows red and white layers; carnelian onyx shows orange and white.
• Colour: true black onyx is scarce. Most black onyx jewellery is grey agate dyed with a sugar-and-acid treatment that carbonises the colour into the stone. It is a stable, long-accepted practice, but the dye can be affected by strong solvents.

What Obsidian Really Is

Obsidian is not a mineral at all but a mineraloid: natural volcanic glass. When silica-rich lava cools so quickly that its atoms never get the chance to arrange themselves into a crystal lattice, the result is a glass — hard, smooth and slightly brittle.

• Hardness: roughly 5 to 5.5 on the Mohs scale, noticeably softer than quartz.
• Fracture: it breaks with a curved, shell-like pattern called conchoidal fracture, producing edges sharp enough that obsidian was long used for blades and arrowheads.
• Appearance: look for gas bubbles, subtle flow lines and a faint translucency at thin edges. There are also snowflake obsidian with grey cristobalite patches, rainbow obsidian with a soft sheen, mahogany obsidian and the rounded Apache tears.

Telling Them Apart in the Hand

If you have both stones loose, a few gentle checks will usually settle the question.

• Look for banding. Onyx often shows straight, parallel layers, even when dyed. Obsidian shows wavy flow lines or tiny bubbles instead.
• Compare the weight. Onyx is denser and feels heavier for its size; obsidian is lighter than it looks.
• Test hardness discreetly. A steel blade will scratch obsidian but not onyx. Try this on the underside of a bead or the back of a cabochon, never on a polished face.
• Warm it in your palm. Glass insulates differently, so obsidian tends to warm up more quickly and feels less cold at first touch.
• Check the edges. Older obsidian pieces often show small chips and nicks along the girdle; onyx holds its edges far better.

Wear, Care and Choosing Between Them

For a ring or bracelet that will see daily knocks, onyx is the safer choice — harder, tougher and far more resistant to scratching and chipping. Its one weakness is the dye used on most black material, so keep it away from acetone, nail varnish remover, bleach and prolonged soaking. Warm water with a drop of mild washing-up liquid and a soft brush is ample, then dry with a lint-free cloth.

Obsidian is better suited to pendants, earrings, brooches and occasional-wear rings. It scratches more easily, chips on impact and should never go into an ultrasonic or steam cleaner. Store it in its own soft pouch rather than loose in a jewellery box, and wipe it with a dry microfibre cloth after wearing to keep the shine.

Both stones share a long history in mourning jewellery and talismanic pieces, and both look superb set in silver, oxidised metal or yellow gold. Choose onyx when you want durability and a deep, even black; choose obsidian when you love the glassy depth and the subtle, shifting character of a volcanic original.