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Material World
Mineral · Al2SiO4(F,OH)2

Topaz

Mohs 8, and the reason there is a step between quartz and corundum — a hard silicate whose main significance is as a reference point and a gemstone.

Gemstone Abrasive

Topaz occupies position 8 on the Mohs scale, between quartz and corundum, and that is much of why it matters. The gap between 7 and 9 is wide, and topaz is the common mineral that sits in it, which is exactly the criterion Mohs used in choosing his ten.

It is an aluminium silicate carrying fluorine and hydroxyl, and it crystallises from the fluorine-rich vapours given off late in the cooling of a granite. Where those vapours have room, the crystals can be enormous.

How to identify it

Topaz is hard — it scratches quartz — and has one perfect cleavage across the crystal, which distinguishes it from the harder corundum and from beryl. That cleavage is also why a topaz can survive years of wear and then split from a single sharp blow.

Colourless, blue, yellow and sherry-brown are the usual colours. Much of the blue topaz sold is colourless material that has been irradiated and heated.

Uses

Topaz is principally a gemstone, valued for hardness and clarity rather than rarity — it is not scarce, and large clean crystals are common enough that size is not the premium it is in other stones.

Industrially it is minor: crushed topaz appears in some abrasive and polishing products, where hardness 8 is enough for the work and corundum would be unnecessary.

How it forms

Topaz forms in pegmatites and in cavities in rhyolite, from fluorine-bearing vapour late in the crystallisation of granitic magma. It occurs alongside fluorite and tourmaline for the same reason: all three take up the fluorine that the main rock-forming minerals reject.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is commonly confused with

  • Quartz — mineral · topaz is harder and has one perfect cleavage; quartz has none and breaks conchoidally

is a component of

  • Pegmatite — rock · an accessory of rare-element pegmatites rather than a defining constituent — from the fluorine-rich vapours late in crystallisation
  • Greisen — rock · crystallised from the fluorine in the altering vapour, and a signature of the process rather than an ore

is an alternative to

  • Aquamarine — mineral variety · as a blue stone: topaz is harder and more often irradiated to reach its blue, aquamarine is naturally blue and softer

contains

  • Aluminium — element Wikidata
  • Fluorine — element · substitutes within a solid solution — an individual specimen may hold little of it Wikidata
  • Hydrogen — element · substitutes within a solid solution — an individual specimen may hold little of it
  • Oxygen — element Wikidata
  • Silicon — element Wikidata

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Questions this page answers

Where it comes from, and what it becomes

Follow Topaz back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Downstream — what it becomes

  • Topaz → is a component of (an accessory of rare-element pegmatites rather than a defining constituent — from the fluorine-rich vapours late in crystallisation) → Pegmatite
  • Topaz → is a component of (crystallised from the fluorine in the altering vapour, and a signature of the process rather than an ore) → Greisen