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Silicate · Nesosilicate (island silicate) · Topaz Group

Topaz

Al₂SiO₄(F,OH)₂

The hardest silicate gemstone and the standard for Mohs 8 — a fluorine-rich pegmatite mineral behind imperial orange, sky blue, and champagne-pink gems.

Hardness8
Crystal systemOrthorhombic
LusterVitreous
Sp. gravity3.49–3.57
ClassSilicate
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Topaz specimen

Topaz — glassy prismatic crystals, the defining mineral of Mohs 8 — Photo: Eric Polk · CC BY-SA 4.0

What is Topaz?

Topaz is an aluminium fluorosilicate — Al₂SiO₄(F,OH)₂ — that forms where fluorine-rich fluids linger in cooling granites and rhyolites. It's the hardest of the common silicate gems: at Mohs 8 it sits exactly halfway between quartz and corundum on the hardness scale, and it defines that step.

Most people meet topaz as an icy blue gemstone. Here's the twist: almost all blue topaz starts life colorless and gets its color from irradiation treatment — natural blue topaz exists, but it's usually pale. The genuinely precious natural colors are the orange-to-pink stones from Ouro Preto, Brazil, sold as imperial topaz.

Fun name quirk: topaz is named for Topazios (Zabargad) Island in the Red Sea — but the gem ancient miners dug there was actually peridot, not topaz at all.

In one sentence: Topaz is hard (Mohs 8), heavy for a pale gem (SG ~3.5), and splits cleanly along one flat basal cleavage — the tell that separates it from quartz and beryl.

Physical Properties

The full data table is in the panel alongside. Here’s what those numbers actually mean.

Hardness. Topaz is the standard for 8 on the Mohs scale — it scratches quartz easily and is in turn scratched by corundum. That hardness, plus good transparency, is why it survives daily wear as a ring stone.

Cleavage. Despite its hardness, topaz has one perfect basal cleavage — a clean internal plane across the base of the prism. Cutters must orient gems so the cleavage never meets a girdle edge, and a hard knock can still split a stone.

Color sources. Colorless is default. Blue comes from radiation-induced color centers (natural or lab); orange, pink and red tones come from chromium; brown 'smoky' topaz holds unstable color centers that can fade in sunlight.

How & Where It Forms

Topaz crystallizes late in the life of fluorine-rich magmas — in the pockets of granitic pegmatites, in greisen (quartz–mica rock altered by hot fluorine vapors), and in gas cavities inside rhyolite lava flows, where it grows directly from mineralizing vapor. Weathering frees the durable crystals into river gravels, where much gem topaz is actually found.

Often grows alongside:

QuartzFeldsparMuscoviteFluoriteBerylTourmalineCassiterite

What It’s Used For

GemstonesBlue topaz (mostly irradiated colorless material) fills the affordable-blue market; imperial orange and pink topaz are the premium gems.
Mohs scale referenceTopaz defines hardness 8 — the step between quartz and corundum.
BirthstoneTopaz is the traditional November birthstone; blue topaz is a modern December birthstone.
Collector specimensGlassy terminated prisms from pegmatites and rhyolite lithophysal cavities are display staples.

How to Identify It

Field-checks: harder than quartz (scratches it), one direction of perfect flat cleavage at the prism base, glassy luster, and a heft that surprises for a pale transparent stone. Orthorhombic prisms are typically striated along their length.

Scratches quartz and glass easily — Mohs 8One perfect basal cleavage across the prism baseNoticeably heavier than quartz (SG ~3.5)Striated orthorhombic prisms with complex terminations
Looks like…But differs because…Quick test
QuartzSofter (7), lighter (SG 2.65), no cleavage, and forms six-sided prismsCleavage & heft
BerylSix-sided prisms without topaz's perfect basal cleavage; lighter (SG ~2.7)Cleavage & density
DanburiteSofter (7–7.5), lighter (SG ~3.0), lacks the clean basal splitHardness & cleavage
FluoriteMuch softer (4) and cleaves in four directions into octahedraScratch & cleavage count

Where It’s Found

Notable localities for fine specimens:

Ouro Preto, Brazil Topaz Mountain, Thomas Range, USA Schneckenstein, Germany Villa de Arriaga, Mexico Volodarsk-Volynskii, Ukraine Mogok, MyanmarOpen the locality map →

References & Further Reading

  1. Dana's New Mineralogy, 8th ed. Topaz group, nesosilicates.
  2. Mindat.org — Topaz mineral data and locality records.
  3. Wikipedia — Topaz.

Last reviewed 2026-09-14 · How to cite this page.