Is dumortierite quartz?
No. Dumortierite is a separate species, AlAl6BSi3O18, hardness 7 to 8.5; quartz is SiO2, hardness 7. Dumortierite quartz is quartz coloured by dumortierite.
No. Dumortierite is an aluminium borosilicate, AlAl6BSi3O18, hardness 7 to 8.5, density 3.21 to 3.41; quartz is SiO2, hardness 7, density 2.65. The blue stone called dumortierite quartz is quartz coloured by dumortierite. A 20.00 g piece of quartz reads 12.45 g in water and dumortierite 13.77 to 14.13 g; the mixture lands between, and the reading shows how much dumortierite is in it.
In short
- Two species, two IMA rows. The September 2026 IMA-CNMNC Master List gives quartz, SiO2, A, 1967 s.p., and dumortierite, AlAl6BSi3O18, Rd, 2013 s.p., France. Worked on standard atomic weights, dumortierite is 31.52% SiO2, 6.09% B2O3 and 62.40% Al2O3 by mass; quartz is 100% SiO2.
- Dumortierite quartz is mostly quartz. The International Gem Society quartz article calls it a dense, deep blue to violet material made up of crystalline quartz coloured by dumortierite, and lists California lapis as a misnomer for it. Its dumortierite article says Arizona dumortierite is actually a quartz-impregnated variety. Choose quartz for the stone, dumortierite for the pure blue fibres.
- The mixing line. Worked as a volume mix of quartz (2.65) and dumortierite (3.21 to 3.41), a 20.00 g piece reads 12.53 to 12.56 g in water at 5% dumortierite, 12.61 to 12.66 g at 10%, 12.76 to 12.86 g at 20% and 13.17 to 13.40 g at 50%. Read backwards, 12.60 g implies about 7 to 9% dumortierite by volume. Pure dumortierite starts at 13.77 g.
- Documented failure: the scratch test. Quartz is hardness 7; dumortierite is 7 to 8.5 on its Handbook of Mineralogy sheet and 7 when massive in the gem society’s figures. Neither reliably scratches the other, and a mixed stone is both at once.
- Documented failure: lapis by weight. GIA’s lapis lazuli page gives lapis lazuli specific gravity 2.75 ± 0.25, which reads 12.00 to 13.33 g for a 20.00 g piece and covers the mixing line from pure quartz to about 50% dumortierite. A blue stone’s weight cannot separate dumortierite quartz from lapis; the lapis tests are on the lapis pages.
| Criterion | Quartz | Dumortierite | Dumortierite quartz (IGS) | Does it choose? |
|---|---|---|---|---|
| What it is | One species, SiO2 | One species, AlAl6BSi3O18 | Crystalline quartz coloured by dumortierite | - |
| IMA row | A, 1967 s.p. | Rd, 2013 s.p., France | None: a trade name | - |
| Silica by mass | 100% | 31.52% ideal | Mostly quartz | Only by analysis |
| Boron | None | 6.09% B2O3 ideal | Only in the dumortierite | Only by analysis |
| Crystal system | Hexagonal | Orthorhombic | Mixed | On crystals only |
| Habit | Prismatic crystals; massive | Fibrous or columnar; parallel needles; massive | Dense, translucent to opaque (IGS) | Partly |
| Hardness | 7 | 7 to 8.5; 7 massive (IGS) | About 7 | No |
| Density | 2.65; 2.59 to 2.63 massive | 3.21 to 3.41 | Between, by proportion | Yes, for pure material |
| 20.00 g piece in water | 12.45 g; 12.28 to 12.40 g massive | 13.77 to 14.13 g | Between: see the next rows | Yes: at least 1.32 g apart |
| Mixture, 5% dumortierite by volume | - | - | 12.53 to 12.56 g | Not on a 0.1 g scale |
| Mixture, 10% | - | - | 12.61 to 12.66 g | Barely |
| Mixture, 20% | - | - | 12.76 to 12.86 g | Yes, against pure quartz |
| Mixture, 50% | - | - | 13.17 to 13.40 g | Yes, against both |
| Refractive index | 1.544 and 1.553; uniaxial (+) | 1.659 to 1.723; biaxial (−) | Quartz readings, with blue fibres | Yes, on an instrument |
| Pleochroism | Rock crystal none (IGS) | Strong: deep blue or violet to colourless (sheet) | Strong blue/violet (IGS) | Partly |
| Colour | Colourless, white; blue among colours from inclusions (sheet) | Smalt-blue to greenish blue, violet, red-violet, purple, brown | Deep blue to violet (IGS) | No |
Is dumortierite quartz? The criteria
Dumortierite is not quartz: it is an aluminium borosilicate with its own IMA row, and only 31.52% of it is silica. The Handbook of Mineralogy dumortierite sheet gives Al7(BO3)(SiO4)3O3, orthorhombic, hardness 7 to 8.5, density 3.21 to 3.41, fibrous or columnar to massive, smalt-blue to violet. The quartz sheet gives SiO2, hexagonal, hardness 7, density 2.65, 2.59 to 2.63 when massive. The IMA list writes dumortierite AlAl6BSi3O18, Rd, 2013 s.p., France, and quartz SiO2, A, 1967 s.p.
Worked composition. On standard atomic weights, AlAl6BSi3O18 is 571.92 g/mol: 31.52% SiO2, 6.09% B2O3 and 62.40% Al2O3. Aluminium makes up 33.02% of the mass, more than silicon’s 14.73%. Quartz has no aluminium or boron in its formula. Why the SiO2 formula alone does not identify quartz is worked on is quartz a mineral and not repeated here; the species pages are quartz and dumortierite.
Is my blue stone dumortierite or quartz? If it is a polished, dense, deep blue to violet stone, the likeliest answer from the sources is both: dumortierite quartz. If it is a mass of blue fibres or needles, it may be dumortierite itself. The table above lists the criteria; the scale, below, measures the mixture.
What is dumortierite quartz? Quartz coloured by dumortierite
Dumortierite quartz is quartz with dumortierite in it, named for the inclusion, the way tourmalinated quartz is. The International Gem Society quartz article describes it as a dense, deep blue to violet material made up of crystalline quartz coloured by dumortierite, a complex borosilicate, and in its variety list as translucent to opaque, coloured dark blue or violet by dumortierite inclusions. It records strong blue/violet pleochroism for it. The International Gem Society dumortierite article says Arizona dumortierite is actually a quartz-impregnated variety; the Handbook’s Distribution field names Clip and Quartzsite, Arizona, and lists quartz first among dumortierite’s associates.
The misnomer. The gem society lists California lapis as a misnomer for dumortierite quartz. Lapis lazuli is a rock, an aggregate of mainly lazurite, calcite and pyrite in GIA’s words; what it is and how its blue mineral behaves is on is lapis lazuli a mineral or rock. The quartz sheet itself says blue is among the colours quartz takes from chemical or particulate inclusions, so blue quartz is not automatically dumortierite quartz; the gem society even puts rose quartz’s colour down to fibrous inclusions of a mineral similar to dumortierite.
The other quartz confusions each have their own page and test: is topaz a quartz, is tourmaline a quartz, is celestite a quartz and is chalcedony quartz. None of their assets is repeated here.
Can a scale tell dumortierite quartz from quartz? The mixing line
A balance reads the proportion of dumortierite in a quartz stone, coarsely. Assume the stone is only quartz (2.65) and dumortierite (3.21 to 3.41), with no pores, and mix by volume: D = (1 − f) × 2.65 + f × 3.21 to 3.41. A 20.00 g piece then reads 12.45 g in water at 0%, 12.53 to 12.56 g at 5%, 12.61 to 12.66 g at 10%, 12.76 to 12.86 g at 20%, 12.90 to 13.05 g at 30%, 13.17 to 13.40 g at 50% and 13.77 to 14.13 g at 100%. Read backwards, 12.60 g implies 6.9 to 9.4% dumortierite by volume, 12.80 g 16.8 to 22.8% and 13.00 g 27.3 to 37.0%.
Where the line fails. A 0.1 g step near 12.45 g equals 4.7 to 6.3% dumortierite, so a kitchen-grade scale cannot see a few per cent. Massive quartz on its sheet is lighter, 2.59 to 2.63, which reads 12.28 to 12.40 g and pulls every rung down. Any third mineral, pore or crack moves the reading too. The line says how much heavier than quartz a stone is; it does not identify the extra mineral. The method is on how to measure specific gravity at home.
Documented failure: lapis by weight. GIA’s lapis lazuli page gives lapis lazuli 2.75 ± 0.25, which reads 12.00 to 13.33 g, which spans the mixing line from pure quartz to about 50% dumortierite, so the scale cannot separate the two blue stones. The 12.00 g line for sodalite is on is sodalite the same as lapis lazuli and is not repeated; lapis tests are on how to tell if lapis lazuli is real.
Why a scratch test fails, and who each name is wrong for
Hardness cannot separate dumortierite from quartz, because both start at 7. Quartz is 7 on its sheet. Dumortierite is 7 to 8.5 on the Handbook sheet, and the gem society gives 8 to 8.5 for crystals but 7 for massive material, the form the blue stones take. Neither reliably scratches the other, and a dumortierite quartz is both at once. The technique is on how to test mineral hardness properly; it settles quartz against softer blue stones such as lazurite (5 to 5.5) and sodalite (5.5 to 6), not this pair.
Quartz is the wrong name for pure dumortierite: blue fibres or needles reading 13.77 g or more for 20.00 g, with refractive indices from 1.659 upwards, point to dumortierite, and the five IMA rows that share its formula block are on dumortierite. Dumortierite is the wrong name for dumortierite quartz: a dense blue stone reading near quartz’s 12.45 g is mostly quartz, and the gem society’s name for it says so.
Lapis is the wrong name for either. California lapis is a misnomer: lapis is a rock of lazurite, calcite and pyrite (GIA), and lazurite prints a bright blue streak on its sheet; no source read here records lazurite or pyrite in dumortierite quartz. Other identification questions are indexed at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.