Black Spinel or Obsidian: How to Tell Them Apart
The most reliable test needs no loupe and no equipment: it's weight. For the same size, black spinel weighs roughly half as much again as obsidian. Here's how to tell these two black stones apart without damaging either one, their properties, five checks you can do at home, and why black spinel is nowhere near a fake version of obsidian.
Two black stones, worlds apart
Obsidian isn't a crystal. It's a natural volcanic glass, born from silica-rich lava that cooled before it could crystallise. Its atoms are frozen in disorder, exactly as in a window pane. It's classed as a mineraloid rather than a mineral, I cover that origin story 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), black varieties owe their colour to a high iron content. It forms in metamorphic rocks and alluvial deposits, mainly in Thailand, Myanmar, Sri Lanka and Madagascar.
This difference in nature explains everything else. A glass is light and soft; a dense crystal from the spinel family is heavy and hard. You don't need to remember the figures that follow: just remember that these two stones only ever resemble each other in colour.
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, iron-rich |
| 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 shows 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 short, two lines cover it in practice. The density (≈ 2.45 versus ≈ 3.60) is immediately obvious in the hand. The hardness explains everything else: why spinel is cut with facets and obsidian isn't, why one scratches and the other doesn't.
Five tests you can do at home
From most to least reliable, and all non-destructive except the last one, which I'd steer you away from.
Weigh it in hand
The decisive test. A bracelet of 22 beads at 8mm weighs around 14.5g in obsidian versus 21g in spinel, nearly 50% more. Hold one bracelet in each hand: the difference is obvious straight away. A small kitchen scale settles it for good.
Check the edges in strong light
Hold the stone up to a window or a bright lamp. On a thin edge or a slim bead, obsidian often lets through a brown, green or grey glow, sometimes with a few trapped gas bubbles. Black spinel, by contrast, stays firmly 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 as a cabochon. A black bead with crisp, sparkling facets? Very likely spinel.
Feel it
A supporting clue, not proof. Both feel cool on first contact, but obsidian, a glass, so a poor conductor, warms up faster in the hand. Spinel stays cold for longer. Cross-check this against the other tests.
Scratch it, best avoided
You'll read everywhere that a steel point (hardness ≈ 5.5) scratches obsidian but not spinel. That's true, and it's also the surest way to permanently damage a piece of jewellery. If you insist, do it on a hidden spot, at the base of a bead. Otherwise, the four tests above are more than enough.
Worth noting, none of these tests replace a proper gemmological examination. For a valuable piece, or a dispute with a seller, only an independent lab can settle it, refractive index and density measurements take just a few minutes there.
Which one should you choose for jewellery?
Worth being upfront: on strictly technical grounds, black spinel wins. At 7.5 to 8 on the Mohs scale, it resists everyday scratches far better than obsidian at 5-5.5. For a ring worn daily, that's a genuine advantage.
Black spinel isn't a fake obsidian. It's a beautiful stone in its own right, harder than mine, and it ages better. The problem was never the stone, it's the label. Selling spinel as "obsidian", or the other way round, is what's unacceptable.
What obsidian has going for it is a different kind of appeal. Its material tells a story: cooled lava, a geological accident, glass born of fire. Its black isn't sparkly but deep, almost liquid depending on the light, an effect spinel's sparkle simply doesn't give. And it's obsidian, not spinel, that crystal healing associates with grounding and protection.
In practice: spinel if you're after sparkle and durability, obsidian if you're after depth and a story. Two different choices, not a right one and a wrong one.
The other black stones people confuse it with
Spinel isn't the only contender. Three other black stones circulate under loosely applied labels:
- Black onyx, a chalcedony, meaning microcrystalline quartz, hardness 6.5 to 7, density ≈ 2.6. The uniform black sold commercially is almost always achieved through dyeing, an accepted practice but one that should be disclosed. It looks more matte and more "solid" than obsidian;
- Black tourmaline (schorl), hardness 7 to 7.5, density ≈ 3.1. Recognisable by its characteristic longitudinal striations, clearly visible on rough pieces. Often sold as tumbled stone rather than as polished beads;
- Manufactured black glass, the only genuine problem in this list. Too perfect, no inclusions, often translucent at the edge, sometimes with a visible mould line. It warms up very quickly in the hand. It's what's behind those "blue obsidians" in impossible colours.
A simple way to sort them: if the stone is faceted and sparkling, it's spinel. If it's matte and perfectly uniform, think dyed onyx. If it's striated, tourmaline. If it's too perfect and lightweight, glass. Obsidian, meanwhile, almost always keeps a small flaw, a bubble, a haze, a shift in tone within the black.
Buying without getting it wrong
Three habits avoid almost every unpleasant surprise.
Ask for a precise name. "Black stone", "black onyx" used loosely, "rainbow obsidian" on a stone with no iridescence at all: a seller who knows their stock names the material without hesitating, and can tell you where it's from. Vagueness about the name is the first red flag.
Be wary of marketing names. "Midnight obsidian", "black diamond" for spinel, "obsidian onyx", these terms exist in no actual classification. They're used to make price comparisons harder. My guide to obsidian price and value sets out the real ranges.
Ask for the weight. It's the most honest piece of information anyone can give you about a bead 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's a question almost no one asks me, and I think that's a shame. Message me and I'll give you the exact weight of the piece you're after: it's the simplest check there is, and a maker who won't answer already tells you plenty.
If you're weighing up the different obsidians, black, gold sheen, snowflake, rainbow, the full picture is in my obsidian varieties guide. And for the complete overview, the full obsidian guide.
Precautions & care
The hardness gap has a very practical 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, abrasive products, ultrasonic cleaners or steam. Take your jewellery off before showering, exercising or sleeping, on a bracelet strung on elastic cord, the cord is the first thing to wear out. It's all covered in cleansing and charging obsidian.
Good to know, the gemmological figures quoted here (hardness, density, refractive index) are measurable data. The properties credited to stones, on the other hand, come from spiritual traditions and crystal healing, a wellbeing practice based on belief. They are not a medical claim, do not treat any condition, and never replace advice from a health professional.
Frequently asked questions
How do you tell black spinel from obsidian?
Is black spinel a fake obsidian?
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Can you scratch-test a piece of jewellery?
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Which black stone is the toughest?
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Genuine obsidian, plain and simple
Bracelets, necklaces, pendants and rings in natural obsidian, handcrafted in France, every variety sold under its true name.