Obsidian, onyx and black tourmaline all read black across a room, and they are three different materials. Obsidian is volcanic glass and the softest of the three, at 5 to 5.5 on the hardness scale. Onyx is banded quartz, about 7. Black tourmaline is a crystal, 7 to 7.5, and the only one of the three that is grooved from top to bottom. On a broken surface those grooves throw blue and violet.
Obsidian, onyx and schorl look almost the same in a photograph. Only one of the three is a crystal.
Search for the difference and most of what you find compares them by what each one is supposed to do for you. That is no help at all when you are holding one and want to know what it is.
This guide compares them by what they are made of and how they behave, which is what you can actually check.
Obsidian and black tourmaline both come through my Tbilisi studio, where I cut and set them by hand. Black tourmaline is the one I shape and then stop before the polish, and that decision turns out to be the whole story.
Schorl, and Why the Surface Shows Colour
The mineral name for black tourmaline is schorl. It is a borosilicate with iron in it, and the iron is what makes it black.
The word tourmaline comes from somewhere else entirely. GIA traces it to the Sinhalese toramalli, mixed gems, the name Dutch merchants gave to the water-worn coloured pebbles they found in the gem gravels of Ceylon, now Sri Lanka.
Schorl is black all the way through. It is opaque, so no light passes through it, and its shine runs from glassy to almost metallic. Mineral references put it as "vitreous to submetallic".
It grows as long prisms with deep grooves running down their length. Those grooves are one of the easiest ways to recognise tourmaline, and neither obsidian nor onyx has anything like them.
Broken faces, not one flat face
I shape each stone and then stop before the polish. That leaves the face broken along those grooves, into hundreds of small planes sitting at slightly different angles.
Each plane sends light back its own way, and that is when the blue, violet and green come up. The colour is in the stone itself. Nothing is sprayed on it and nothing is added to it. What the broken surface does is bring it out.
Tilt the piece and the colour moves across it. Hold it still and it reads black again.
Polishing flattens all of it
A polish takes those small planes and grinds them into one. One plane returns one reflection, and one reflection off a black mineral is flat black.
That is why polished black tourmaline looks so different. Nothing is wrong with either stone. The two surfaces simply do different things with light.
Three Black Stones, Told Apart
They are not related to each other. One is glass, one is quartz, one is a crystal.
| Obsidian | Black onyx | Black tourmaline | |
|---|---|---|---|
| What it is | Volcanic glass | Banded quartz | Crystal |
| Hardness | 5 to 5.5 | About 7 | 7 to 7.5 |
| Light at a thin edge | Passes, often brown | Stops | Stops |
| Pattern | None | Bands across | Grooves down the length |
| Colour off the surface | None | None | Blue and violet on a broken face |
| Usually dyed | No | Yes | No |
Most black onyx is dyed. The Gem Society states it plainly: dyeing is so common that most gem certificates note black onyx is "probably dyed", and the colour reaches "only to a depth of a few millimeters".
That is not a reason to avoid onyx. It is a reason to know what you are looking at, because black tourmaline never needs it. Schorl is black on its own, from the iron inside it.
Three checks you can make
Hold a thin edge to a lamp. Obsidian lets light through where it is thin, usually as a warm brown. Onyx and tourmaline both stop it. I wrote about that test and five others in real versus fake obsidian.
Look for a direction. Tourmaline grooves run one way, down the length of the stone. Onyx bands run straight across it. Obsidian has neither, because glass never grew as a crystal.
Tilt it under a lamp. This is the one only tourmaline does. Blue and violet come up off the broken face and travel as you move your hand. Obsidian stays black with one hard bright point of light, and onyx looks the same from every angle.
Beads and Claws, Not a Bezel
A bezel is a smooth collar of silver folded over the edge of a stone. It is the neatest way to hold one, and it needs an edge that is even the whole way round.
A stone that has been shaped and stopped short of the polish does not have that edge. The rim is broken, the same as the face.
So I hold these stones another way. On the rings, small silver beads stand up around the rim and grip the stone from the edge. On the earrings the beads run all the way round and a few claws reach in over the face as well. On the pendants it is either claws closing over the edge, or a frame of silver branches built around the stone. Every one of them holds from the outside, which is what a broken edge needs.
Amethyst needs the same answer for a different reason. There the crystal points stand higher than the rim, and I wrote about that stone in how to tell if amethyst is real.
The stone itself is not delicate. At 7 to 7.5 it is as hard as quartz or a little harder, and well above obsidian, so it takes daily wear without trouble. The setting is not there to protect a fragile stone. It is there because of the surface I chose to keep.
Three Questions Before You Buy
Look at the surface in the photograph first. A broken face with a direction in it will show colour. A flat, even face will read black. Both are black tourmaline, finished differently, and photographs of the two look nothing alike.
Ask which one you are getting. A seller who shaped the stone can tell you.
Ask what holds it. That surface needs beads or claws to grip it, so a smooth bezel over one is worth asking about.
Mine are shaped by hand in my Tbilisi studio and set in 925 silver, each stone chosen one at a time. You can see them in my black tourmaline collection.
Colour in a stone almost always comes from something inside it, and I found the same thing when I looked at quartz: the cloud in rose quartz is the same thing that makes it pink.
Common Questions About Black Tourmaline
Is black tourmaline rare?
No. Schorl is the most common tourmaline there is: one account puts it at "95% or more of all tourmaline in nature". What costs money in a finished piece is the cutting and the silver work, not scarcity.
Why does my black tourmaline collect dust?
Because it is pyroelectric. Warm it, even just by wearing it, and it takes a small electric charge that pulls in dust and lint. IGS puts it plainly: these gems "will attract more dust than non-conductive stones". A soft brush is the answer.
Will the colour fade?
No. It is not a dye and not a coating. The blue and violet belong to the stone and show on the broken surface, so there is nothing to wear away. A polish would flatten that surface, so I do not take it that far.
Can it get wet?
Yes. Warm water, mild soap and a soft brush are all it needs, then dry it. Keep it out of ultrasonic cleaners. The brush matters more here than with a smooth stone, because a broken surface holds more dust.
Is schorl the same as black tourmaline?
Yes. Schorl is the mineral name and black tourmaline is what the same stone is called in jewellery.
