Black Spinel or Obsidian: How to Tell Them Apart
The most reliable test needs no loupe and no equipment: it is weight. At an equal size, black spinel weighs roughly half as much again as obsidian. Here is how to tell these two black stones apart without damaging either, their constants, five checks you can do at home, and why black spinel is not remotely a fake version of obsidian.
Two black stones with nothing else in common
Obsidian is not a crystal. It is a natural volcanic glass, born from a silica-rich lava that froze before it could crystallise. Its atoms are locked in disorder, exactly as in a pane of glass. It is classed among the mineraloids, not the minerals, I go into that origin in how obsidian is formed.
Black spinel is the opposite in every respect: a true mineral, crystallised in the cubic system, with the formula magnesium aluminate (MgAl₂O₄), the black varieties owing their colour to a high iron content. It forms in metamorphic rocks and alluvial deposits, mainly in Thailand, Myanmar, Sri Lanka and Madagascar.
That difference in nature explains everything else. A glass is light and comparatively soft; a dense crystal of the spinel family is heavy and hard. You do not need to remember the figures that follow: just remember that these two stones resemble each other in colour alone.
Obsidian and black spinel: the comparison table
| Criterion | Obsidian | Black spinel |
|---|---|---|
| Nature | Volcanic glass, amorphous | Crystalline mineral, cubic |
| Composition | Silicate glass, over 70% SiO₂ | MgAl₂O₄ rich in iron |
| Hardness (Mohs) | 5 to 5.5 | 7.5 to 8 |
| Density | 2.35 to 2.60 | 3.58 to 4.06 (≈ 3.60) |
| Refractive index | 1.48 to 1.51 | 1.71 to 1.74 |
| Lustre | Vitreous | Vitreous to sub-adamantine |
| On thin edges | Often translucent brown, green or grey | Stays opaque black |
| Common cut | Round beads, cabochons, discs | Faceted beads, rondelles |
| Origin | Mexico, USA, Iceland, Italy | Thailand, Myanmar, Sri Lanka, Madagascar |
In plain terms, two lines are enough in practice. The density (≈ 2.45 versus ≈ 3.60) is felt immediately in the hand. The hardness explains everything else: why spinel is cut with facets and obsidian is not, why one scratches and the other does not.
Five tests you can do at home
From most reliable to least, and all non-destructive except the last, which I would steer you away from.
Weigh it in your hand
The decisive test. A bracelet of 22 beads at 8 mm weighs around 14.5 g in obsidian against 21 g in spinel, nearly 50% more. Hold one bracelet in each hand: the difference is obvious. A small kitchen scale settles it for good.
Check the edges in bright light
Hold the stone up to a window or a strong lamp. On a thin edge or a slim bead, obsidian often lets through a brown, green or grey glow, sometimes a few frozen gas bubbles. Black spinel, by contrast, stays resolutely opaque.
Look at the cut
The quickest shortcut. Black spinel is almost always faceted: its hardness lets it hold sharp, bright edges. Obsidian, being softer, is worked round or into cabochons. A black bead with crisp, sparkling facets? Very likely spinel.
Touch it
A supporting clue, not proof. Both feel cool on first contact, but obsidian, a glass, and so a poor conductor, warms up faster in the hand. Spinel stays cool for longer. Cross-check this against the other tests.
Scratch it, best avoided
You will read everywhere that a steel point (hardness ≈ 5.5) scratches obsidian but not spinel. That is true, and it is also the surest way to permanently damage a piece of jewellery. If you insist, do it on a hidden area, at the base of a bead. Otherwise, the four tests above are more than enough.
Careful, none of these tests replaces a gemmological examination. For a valuable piece, or in a dispute with a seller, only an independent laboratory can settle it, refractive index and density measurements take a matter of minutes there.
Which one should you choose for jewellery?
Best to be honest: on strictly technical grounds, black spinel wins. At 7.5 to 8 on the Mohs scale, it resists everyday scratches far better than an obsidian at 5-5.5. For a ring worn daily, that is a real advantage.
Black spinel is not a fake obsidian. It is a beautiful stone, harder than mine, that ages better. The problem was never the stone, it was always the label. Selling spinel under the name "obsidian", or the reverse, is what is unacceptable.
What obsidian has in its favour is a different kind of thing. Its material tells a story: frozen lava, a geological accident, glass born of fire. Its black is not sparkling but deep, almost liquid depending on the light, a look spinel's sparkle simply does not give. And it is obsidian, not spinel, that crystal healing links to grounding and protection.
In practice: spinel if you are after sparkle and durability, obsidian if you are after depth and a story. Two different choices, not a right one and a wrong one.
Other black stones people confuse
Spinel is not the only candidate. Three other black stones circulate under loose labels:
- Black onyx, a chalcedony, so microcrystalline quartz, hardness 6.5 to 7, density ≈ 2.6. The uniform black sold commercially is almost always achieved by dyeing, which is an accepted practice but ought to be disclosed. A more matt, more "solid" look than obsidian;
- Black tourmaline (schorl), hardness 7 to 7.5, density ≈ 3.1. Recognisable by its characteristic lengthwise striations, clearly visible on raw pieces. Often sold as a tumbled stone rather than as polished beads;
- Manufactured black glass, the only genuine problem in the lot. Too perfect, no inclusions, often translucent at the edge, sometimes with a mould line. It warms up very quickly in the hand. This is what lies behind the "blue obsidians" sold in impossible colours.
A simple way to sort them: if the stone is faceted and sparkling, think spinel. If it is matt and perfectly uniform, think dyed onyx. If it is striated, tourmaline. If it is too perfect and too light, glass. Obsidian, meanwhile, almost always keeps a small flaw, a bubble, a faint veil, a shift of tone within the black.
Buying without getting it wrong
Three habits avoid nearly all the nasty surprises.
Ask for a precise name. "Black stone", "black onyx" used loosely for anything dark, "rainbow obsidian" on a stone that does not iridesce: a seller who knows their stock names the material without hesitating, and can tell you where it comes from. Vagueness about the name is the first warning sign.
Be wary of marketing names. "Midnight obsidian", "black diamond" for what is actually spinel, "obsidian onyx", these names exist in no nomenclature. They serve to make price comparisons harder. My guide to obsidian price and value sets out real price ranges.
Ask for the weight. This is the most honest piece of information anyone can give about a beaded bracelet, and the one most rarely offered. Combined with the bead diameter and count, it lets you check the density yourself in thirty seconds.
It is a question almost no one asks me, and I find that a shame. Write to me and I will give you the exact weight of the piece you are interested in: it is the simplest check there is, and a maker who refuses to answer has already told you something.
If you are torn between the different obsidians, black, gold sheen, snowflake, celestial eye, the full picture is in my guide to the types of obsidian. And for the overview, the complete obsidian guide.
Precautions & care
The difference in hardness has one very concrete consequence: never store your two stones together. Spinel, at 8 on the Mohs scale, scratches obsidian effortlessly. Each piece deserves its own fabric pouch or compartment.
For obsidian: a soft microfibre cloth, a brief rinse in lukewarm water if needed, never salt, never an abrasive product, never an ultrasonic cleaner or steam. Take your jewellery off before showering, exercise and bed, on a bracelet strung on elastic thread, it is the thread that wears out first. Everything is set out in cleaning your obsidian, and the energetic side in cleansing and charging.
Worth knowing, the gemmological constants cited here (hardness, density, refractive index) are measurable data. The properties credited to stones, by contrast, belong to spiritual traditions and to crystal healing, a wellbeing practice founded on belief. They are not a medical claim, they treat no illness, and they are never a substitute for the advice of a healthcare professional.
Frequently asked questions
How do you tell black spinel from obsidian?
Is black spinel a fake obsidian?
What is the hardness of obsidian and spinel?
Can you do a scratch test on a piece of jewellery?
How do you tell black onyx from obsidian?
Which black stone is the most durable?
How can you check a bracelet's density?
Genuine obsidian, plain and simple
Bracelets, necklaces, pendants and rings in natural obsidian, handmade in France, every variety sold under its true name.