Gem library
Topaz
Aluminium silicate with a formidable cleavage
Topaz is one of the few gems to reach hardness 8, yet it has a perfect basal cleavage that makes it paradoxically fragile. Its blue version, ubiquitous and cheap, is almost always obtained by irradiating a colourless topaz.
- Hardness (Mohs)
- 8
- Density
- 3.49 – 3.57
- Crystal system
- Orthorhombic
- Refractive index
- 1.609 – 1.643
- Cleavage
- Perfect on {001}
Mineralogical identity
- Family
- Silicates — nesosilicates
- Species
- Topaz
- Variety
- Imperial topaz, Sherry topaz, London Blue topaz, Mystic topaz
- Chemical formula
- Al₂SiO₄(F,OH)₂
- Crystal system
- Orthorhombic
- Classification
- Semi-precious stone
- Rarity
- Uncommon
- Indicative price
- €5 – 80/ct (blue) · €200 – 1,500/ct (imperial)
Etymology
From the Greek Topazos, the name of an island in the Red Sea (today Zabargad) — which in fact produced peridot, not topaz.
Physical & optical properties
- Hardness (Mohs)
- 8
- Density
- 3.49 – 3.57
- Refractive index
- 1.609 – 1.643
- Birefringence
- 0.008 – 0.011
- Cleavage
- Perfect on {001} — basal, very clean
- Fracture
- Conchoidal
- Lustre
- Vitreous
- Diaphaneity
- Transparent
- UV fluorescence
- Weak
- Pleochroism
- Weak
- Colours
- blue, yellow, colourless, pink, orange, brown
Formation, geology & history
Hard but cleavable
Topaz occupies step 8 of the Mohs scale, between quartz and corundum, and would therefore scratch any common stone. But it has a perfect basal cleavage perpendicular to the c axis: a well-placed knock splits it cleanly, like a log. Lapidaries know this and orient the table slightly off the cleavage plane to limit the risk. In a ring, topaz must be set protectively, and lateral impacts avoided. This combination — very hard yet brittle — is the best illustration of the difference between hardness and toughness, two properties often confused.
The colours and their treatments
The commonest natural topaz is colourless, followed by pale yellow and brown. Natural blue topaz exists but stays very pale: all the vivid blues of the trade — Sky Blue, Swiss Blue, London Blue — come from colourless topaz irradiated then heated. The treatment is stable, legal and universally practised since the 1970s, which has made blue topaz extraordinarily affordable. At the other end, imperial topaz, orange-pink to raspberry pink, is natural and rare: it comes practically only from Ouro Preto in Brazil and reaches several hundred euros per carat. «Mystic» and «Azotic» topazes with rainbow reflections are colourless topazes coated with a surface metallic layer, a coating that wears with time.
Records and confusions
Topaz forms gigantic crystals in pegmatites: the National Museum of Brazil held a 596 kg crystal, and the largest known faceted topaz, «El Dorado», exceeds 31,000 carats. Terminologically, topaz has been the victim of a historic swap: its name comes from an island that produced peridot, and for centuries anything yellow was called topaz — hence the misleading trade names «Madeira topaz» for heated quartz or «smoky topaz» for smoky quartz. The term topaz should be reserved for the true mineral species.
How to recognise it
- High density for a pale stone3.5: topaz is markedly heavier than a quartz of the same size. That is the first discriminator against a «topaz» that is not one.
- Visible basal cleavageOn antique or rough stones, a perfectly flat, shiny fracture plane perpendicular to the crystal's length marks it as topaz.
- Scratches quartzHardness 8: it marks an amethyst without effort. A quartz sold as topaz fails this test.
Imitations, treatments & confusions
- Quartz sold as topaz«Madeira topaz», «smoky topaz», «gold topaz» in fact denote heated or smoky quartz. Density and hardness tests: quartz is lighter and does not scratch amethyst.
- Blue glassBubbles, softer lustre, no birefringence, low density. Common in costume jewellery imitating blue topaz.
- Undeclared irradiated topazNot a fake, but the information must appear. A bright blue topaz at a low price is necessarily treated: intense natural blue topaz practically does not exist.
Deposits & origins
Brazil (Ouro Preto, Minas Gerais — the only significant source of imperial topaz), Russia (Urals), Pakistan (pink topaz from Ghundao Hill), Nigeria, Madagascar, Sri Lanka, Mexico, United States (Utah, Colorado)
Care & precautions
- Warm soapy water and a very soft brush. Rinse and dry carefully.
- Never ultrasonic cleaning or steam: vibration exploits the basal cleavage and splits the stone. It is the classic workshop accident.
- Avoid prolonged sunlight: yellow, imperial and pink topazes fade in light.
- Store separately and avoid lateral impact: hardness 8 but real fragility.
Traditions & crystal healing
- Chakras
- Solar plexus · throat
- Zodiac signs
- Sagittarius · Scorpio
- Element
- Fire · Air
- Month
- November · December (blue)
Tradition credits it with clarity of mind and kindly self-assertion: yellow topaz is said to be the stone of radiant confidence, blue topaz of calm speech. Medieval lapidaries believed it could detect poisons, and it figured among the twelve stones of Aaron's breastplate.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Is my blue topaz natural?
Almost certainly irradiated. Natural blue topaz exists but stays very pale. All vivid Sky, Swiss and London Blues are obtained by irradiation then heating — a stable and legal treatment.
Why did my topaz split?
Because of its perfect basal cleavage. Despite hardness 8, a knock or a pass through an ultrasonic cleaner can split it cleanly along a flat plane. That is not a defect but its nature.
What is imperial topaz?
The natural orange-pink to pink variety, almost exclusively from Ouro Preto in Brazil. It is the only genuinely rare and expensive topaz: several hundred euros per carat against a few euros for treated blue.