Smoky Quartz Meaning and Properties: A Jewellery Guide
Smoky quartz is the grey to brown colour variety of quartz. The aluminium colour centres behind it, Mohs 7 hardness, and how it reads against obsidian.
In this field note
Smoky quartz belongs to the same quartz family as amethyst, citrine and clear quartz. The colour and the mechanism behind it are what separate them.
What smoky quartz is
Smoky quartz is quartz with a grey, brown or near black body colour. The mineral species is Quartz, the formula is SiO₂, and at room temperature it crystallises in the trigonal system. The usual habit is a six-sided prism. Rounding that prism into a 10mm sphere changes the outside and leaves the crystallography underneath exactly as it was.
Several names describe the same material. Cairngorm is a traditional trade name that the gem trade still accepts. Brown quartz covers the lighter, tea-toned end. Morion names very dark brown to black smoky quartz; it is a description of depth, and it stays within the quartz species. Smoky topaz is a name to avoid, because it labels quartz as topaz. Topaz is a different mineral with a different formula and hardness.
What gives smoky quartz its colour
The brown comes from a defect in the crystal structure called a colour centre. As quartz grows, aluminium occasionally takes a silicon site in the lattice. Ionising radiation in the surrounding rock then acts on those aluminium sites and produces [AlO₄]⁰ centres. Those centres absorb part of the visible spectrum, and what passes through is the brown you see.
Two things follow from that. The colour lives in the structure, so a smoky bead holds its translucency: hold it to a window and light comes through the dark. And the crystal can finish growing before any colour appears, because the radiation does its work on a quartz crystal that is already there. The mechanism is set out in full in how natural radiation creates the brown.
Heat runs the process backwards. At around 200 °C the relevant centres begin to decay and the smoke lightens. Response to ordinary daylight varies from stone to stone, so keep a strand out of long, direct sun.
Smoky quartz properties and hardness
Smoky quartz hardness is Mohs 7, the figure for all macrocrystalline quartz. Specific gravity is about 2.65. There is no cleavage, so the stone breaks along curved conchoidal surfaces. The hardness chart places that 7 beside the other stones BE. strings.
| Property | Smoky quartz |
|---|---|
| Species | Quartz, a colour variety of it |
| Formula | SiO₂ with trace aluminium |
| Crystal system | Trigonal |
| Mohs hardness | 7 |
| Specific gravity | About 2.65 |
| Cleavage | None; conchoidal fracture |
| Body colour | Grey, brown or near black |
| Transparency | Transparent to translucent |
| Colour cause | Aluminium colour centres activated by irradiation |
| Other accepted names | Cairngorm, brown quartz, Morion for very dark material |
Mohs 7 covers the everyday abrasion a strand meets against desks, sleeves and door handles. A hard knock on a stone step can still chip a bead.
Smoky quartz, brown quartz and obsidian
Brown quartz is the same material as smoky quartz under a name that suits the lighter tones. Obsidian is a different material with a different history. Polished beads of all three can read as dark brown to black from a metre away.
| Point | Smoky quartz | Brown quartz | Obsidian |
|---|---|---|---|
| Material class | Crystalline mineral | Crystalline mineral, same species | Natural volcanic glass |
| Identity | Quartz, SiO₂ | Quartz, SiO₂ | Silica-rich glass, variable chemistry |
| Internal structure | Ordered trigonal lattice | Ordered trigonal lattice | Amorphous glassy matrix |
| Mohs hardness | 7 | 7 | About 5 to 5.5 |
| Light through a bead | Brown to grey, transparent to translucent | Lighter tea brown, usually translucent | Commonly opaque; light at a thin edge in some varieties |
| Status of the name | Accepted species name plus colour | Accepted trade name for the same quartz | Rock name, one step above mineral level |
Light separates them in the hand. Quartz carries brown light through the body of a bead, and the tone has depth you can look into. Obsidian is commonly opaque, with light appearing at a thin rim. The full side by side is in smoky quartz vs obsidian.
Two smoky variants have their own colour story. Blue smoky quartz takes its blue haze from Tyndall scattering inside the crystal, with blue needles visible in the beads as well, covered in the blue smoky quartz guide. Smoky rutilated quartz is a smoky host holding fine needle inclusions.
Smoky quartz vs citrine
Smoky quartz and citrine are two colour varieties of one species with two separate colour causes. The yellow of citrine is iron related and sits throughout the body of the quartz. Smoky grey and brown come from the aluminium colour centres described above. Both stones are Mohs 7 and both are SiO₂.
Heat links them in one direction. Enough heat destroys the smoky colour centres, and the smoke goes. What remains depends on the material: pale smoke, colourless quartz, or yellow in material that carries the right iron chemistry. A smoky stone carries no promise of citrine. The natural versus heated citrine guide explains how colour and treatment are assessed.
Choosing and wearing a smoky quartz strand
Three readings matter on a smoky quartz strand: how deep the tone runs, whether light still travels through it, and how closely the beads match each other. BE. scores the first two as Tone and Transparency under Crystal 4T™. The Smoky Quartz Strand Bracelet reads Exceptional on Transparency and Exceptional on Tone, with Strong on Texture, the consistency of the sphere polish. The tier follows the lowest of those three readings, which puts the strand at Signature.
The strand is 10mm beads on a stainless steel wire core, closed with a one-hand magnetic clasp. The stone is smoky quartz from Brazil and it is untreated: no heat, no dye, no coating, no artificial irradiation. Artificial irradiation is a known commercial treatment for this variety, which is why the treatment line is recorded for each lot on the Stone Origin Record.
Wipe the beads with a soft cloth and lukewarm water. Keep the strand away from harder stones in a shared box. Store it away from long, direct sunlight and away from heat.
Frequently asked questions
Q1. What is smoky quartz?
Smoky quartz is the grey to brown colour variety of quartz, SiO₂. The mineral species is Quartz, the same as clear quartz and amethyst.
Q2. What gives smoky quartz its colour?
Aluminium takes a silicon site in the quartz lattice, and ionising radiation then forms [AlO₄]⁰ colour centres around that substitution. Those centres absorb part of the visible spectrum and leave a brown to grey body colour.
Q3. Is smoky quartz natural?
Natural smoky quartz forms where radiation in the surrounding rock acts on the crystal over geological time. Artificial irradiation is a known commercial treatment that produces a similar colour, so the treatment line comes from the lot record. BE.'s smoky quartz is untreated. The mechanism and the terminology are set out in how natural radiation creates the brown.
Q4. What is the hardness of smoky quartz?
Mohs 7, with a specific gravity of about 2.65. Quartz has no cleavage, so it breaks along curved conchoidal surfaces, and a hard knock can chip a bead.
Q5. Can smoky quartz turn into citrine?
Heat destroys the smoky colour centres. The colour left behind depends on the material and can be paler smoke, colourless quartz or yellow. Smoky quartz carries no promise of citrine.
Q6. Is Morion a different stone?
No. Morion names very dark brown to black smoky quartz, and the species stays Quartz. The depth of the colour describes how the stone looks and says nothing about its treatment history.
References
Field Notes.
One stone at a time: its geology, and how to read quality. The Stone Buying Checklist comes with your first email.
By subscribing you agree to receive emails from BE. Unsubscribe anytime.
Sent. Check your inbox.