Gem library
Dumortierite
A deep blue born in the Rhône
Dumortierite is an aluminium borosilicate of deep blue to violet, discovered and named in France. It occurs most often as fine needles included in quartz, forming the blue «quartz-dumortierite» that jewellery sometimes sells as a lapis lazuli substitute.
- Hardness (Mohs)
- 7
- Density
- 3.26 – 3.41
- Crystal system
- Orthorhombic
- Refractive index
- 1.659 – 1.692
- Cleavage
- Distinct on {100}
Mineralogical identity
- Family
- Dumortierite — borosilicate group
- Species
- Dumortierite
- Variety
- Quartz-dumortierite (aggregate)
- Chemical formula
- Al₇BO₃(SiO₄)₃O₃
- Crystal system
- Orthorhombic
- Classification
- Collector's mineral · ornamental stone
- Rarity
- Common
- Indicative price
- €3 – 20/ct
Etymology
Named in 1881 by Ferdinand Gonnard in honour of Eugène Dumortier, a palaeontologist from Lyon. The type locality is Chaponost, in the Rhône department of France.
Physical & optical properties
- Hardness (Mohs)
- 7
- Density
- 3.26 – 3.41
- Refractive index
- 1.659 – 1.692
- Birefringence
- 0.015 – 0.027
- Cleavage
- Distinct on {100}
- Fracture
- Uneven to fibrous
- Lustre
- Vitreous to dull
- Diaphaneity
- Opaque to translucent
- UV fluorescence
- None
- Pleochroism
- Distinct
- Colours
- blue, violet, indigo, brown
Formation, geology & history
Formation and geology
Dumortierite is a mineral of contact metamorphism and of boron- and aluminium-rich pegmatites. It appears in gneiss, mica schist and quartzite, as sheaves of very fine fibrous needles, rarely as visible isolated crystals. The type locality at Chaponost, near Lyon, introduced it to mineralogy in 1881; France does not have many type localities for gem minerals, and this one deserves mention. Its colour, from Prussian blue to violet, comes from titanium and iron substituting for aluminium; a pink and a brown variety also exist, less sought after.
Quartz-dumortierite
What circulates in the trade as dumortierite is almost always massive quartz shot through with myriad blue dumortierite needles, mined mainly in Bahia, Brazil. The result is a deep blue stone, often zoned with white and violet, opaque to translucent, that takes a fine polish. Its effective hardness is that of quartz — 7, pure dumortierite reaching 8.5 — which makes it markedly more robust than most opaque blue stones: lapis lazuli is 5-6, sodalite 5.5-6. It is an excellent choice for cabochons, beads and objects meant to last.
An honest alternative to lapis
Sold under its own name, dumortierite is an appealing stone: harder than lapis, indifferent to water, non-porous, and much cheaper. The trouble comes from vocabulary: it turns up labelled «Brazilian lapis», «azure stone» or «California lapis», names that do not exist and serve to obtain a lapis price for a stone that is not one. True lapis is a lazurite rock, often flecked with golden pyrite, softer and porous. Dumortierite never contains pyrite and shows a fine fibrous felting under the loupe.
How to recognise it
- Fibrous felting under the loupeAt 10×, blue needles can be seen interwoven through colourless quartz, like felt. Lapis is granular, with lazurite grains and often pyrite.
- No pyriteDumortierite contains no golden flecks. A deep blue flecked with metallic gold is lapis lazuli; a deep blue with no flecks at all is more likely dumortierite or sodalite.
- Quartz hardnessQuartz-dumortierite scratches glass effortlessly and resists steel. Lapis and sodalite are marked by a steel point. It is the cleanest test.
Imitations, treatments & confusions
- Dumortierite sold as lapis lazuliThe commonest substitution, under invented names. Look for pyrite (absent in dumortierite), test the hardness, and examine the structure: fibrous here, granular in lapis.
- Dyed jasper and «Swiss lapis»«Swiss lapis» is jasper dyed blue with ferrocyanide: the dye shows in the fissures and the colour is too even. It is neither lapis nor dumortierite.
- Sodalite confusedSodalite is a genuine blue mineral, softer, often veined white in broad clean patches. Dumortierite shows a more violet blue and a fibrous felting, without clear veining.
Deposits & origins
France (Chaponost, Rhône — type locality; Beaujolais), Brazil (Bahia, Minas Gerais — the blue quartz-dumortierite of the jewellery trade), Madagascar, Namibia, Poland, Norway, Sri Lanka, United States (Nevada, Arizona, California), Peru
Care & precautions
- Warm soapy water and a soft brush: quartz-dumortierite is robust and does not fear water.
- Ultrasonic cleaning generally acceptable on a sound piece, to be avoided where fractures are present.
- Indifferent to light; the colour, mineral rather than dyed, does not shift.
- Store without special precaution, away from topaz and corundum.
Traditions & crystal healing
- Chakras
- Third eye · throat
- Zodiac signs
- Sagittarius · Virgo
- Element
- Air · Water
- Month
- September
Crystal healing links dumortierite to the third eye and to speech: a stone of patience and mental discipline, said to support learning, organisation and calm in the face of agitation. Having no ancient tradition of its own, it has inherited much of the symbolism of the lapis lazuli it resembles — which says a great deal about how meanings are born.
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
Dumortierite or lapis lazuli?
Lapis contains lazurite and often golden pyrite, and is marked by steel. Dumortierite is quartz loaded with blue needles, harder, pyrite-free, with a fibrous felting visible under the loupe.
Can it go in water?
Yes, without restriction: hardness 7, no porosity, no workable cleavage in the quartz aggregate. That is one of its advantages over lapis, which is porous and soft.
Is it a French stone?
Its type locality is indeed French — Chaponost, in the Rhône — and its name honours a scholar from Lyon. The dumortierite of the trade, however, almost always comes from Brazil or Madagascar.