Obsidian and What It Gets Confused With: Black Agate, Black Tourmaline, Onyx

Why this matters

Black stones are the easiest category to mislabel, because the feature most people use to identify a stone, its colour, is identical across all of them. Obsidian, black agate, onyx and black tourmaline are four different materials routinely sold under one another's names. Sometimes that's a mistake. Sometimes it isn't.

They differ in weight, in how they break, in how they feel, and in what it costs to produce them.

Obsidian

What it is: volcanic glass. Not a mineral.

How to tell: it's the only glass in the group. Hold a thin edge up to a strong light and most obsidian shows a faint brown or smoky translucency. It fractures into smooth curved surfaces. It's usually noticeably lighter than agate of the same size.

In the hand: glassy, slightly waxy if it hasn't been polished.

Black agate

What it is: a crystalline form of quartz, with an ordered internal structure. A mineral.

How to tell: agate is chalcedony, and chalcedony is denser than obsidian, which you notice immediately when you pick it up. It doesn't produce those smooth curved fracture surfaces. Under a lens it reads opaque; obsidian keeps that thin-edge translucency.

In the hand: colder on first contact, and heavier.

Onyx

What it is: properly, a banded agate with flat parallel layers. In commercial jewellery the word now covers almost any uniform black chalcedony.

How to tell: real banded onyx shows flat parallel layers. A lot of what's sold as onyx is dyed agate, and the dye is the tell. It tends to collect in cracks, so the colour sits slightly differently at the edges of a break than it does in the body of the stone.

In the hand: smooth and heavy, much like agate.

Black tourmaline

What it is: a separate mineral with its own crystal structure, usually found as striated prisms.

How to tell: look for lengthwise striations running along the crystal faces. Tourmaline is harder than obsidian and resists scratching better. It's also pleochroic, meaning the colour shifts slightly depending on the angle you view it from. Obsidian doesn't do that.

In the hand: cooler and harder, with visible linear structure.

Two checks that work almost every time

Weight. Obsidian is the lightest of the four. If a stone of a given size feels heavier than you expected, it isn't obsidian.

Edge translucency. Hold a thin edge up to a bright light. A bead hole, a chip, a corner. Obsidian often shows a faint warm translucency. The other three don't.

Why we bring it up

We sell obsidian as obsidian, and we'd rather you knew what you were buying before you bought it, including somewhere else. If a listing can't tell you which of these four you're getting, that tells you something too.