Danburite
Danburite is calcium borosilicate, CaB2Si2O8: orthorhombic, hardness 7 to 7.25, density 2.93 to 3.02. A refractometer takes it for topaz; a balance does not.
Danburite is a calcium borosilicate, CaB2Si2O8: orthorhombic, hardness 7 to 7.25, density 2.93 to 3.02, named for Danbury, Connecticut. Its refractive indices, 1.627 to 1.639, sit inside topaz’s range, so a refractometer cannot separate them. A balance can: a 20.00 g piece reads 13.17 to 13.38 g in water, topaz 14.27 g or more and quartz 12.45 g.
In short
- One formula, two ways of writing it. The September 2026 IMA-CNMNC Master List lists danburite as CaB2Si2O8, status G, 1839, USA; the Handbook of Mineralogy sheet writes CaB2(SiO4)2. On standard atomic weights that is 28.32% B2O3; the sheet’s Maglovec Hill, Slovakia analysis prints 27.65%.
- The middle-weight rule. A 20.00 g piece of danburite reads 13.17 to 13.38 g in water on its sheet’s densities, 2.93 to 3.02. Quartz, at 2.65, reads 12.45 g and topaz, at 3.49 to 3.57, 14.27 to 14.40 g. Danburite sits between them with 0.72 g clear below and 0.89 g clear above.
- Where the refractometer fails, documented. The danburite sheet’s indices, α 1.627 to γ 1.639, lie wholly inside the topaz sheet’s, α 1.606 to γ 1.644. Both are orthorhombic and biaxial, and the International Gem Society describes danburite crystals as prismatic with wedge-shaped terminations, like topaz. Topaz has perfect {001} cleavage; danburite’s is indistinct.
- The B2Si2O8 trio. A script over the IMA list finds 3 rows whose formula ends B2Si2O8: danburite (Ca), pekovite (Sr) and maleevite (Ba), the last two from Tajikistan, IMA numbers 2003-035 and 2002-027. On their sheets pekovite reads 3.35 and maleevite 3.78: 14.03 and 14.71 g in water, both heavier than any danburite.
- Fluorescence: not on the sheet. A ligature-aware search of the Handbook danburite sheet finds no luminescence wording, and this page gives none. Danburite occurs in granite and metamorphosed carbonate rocks with hydrothermal activity, and in evaporites, with titanite, axinite, tourmaline and quartz among its associates.
| Material, source | Formula (IMA) | Optical class | Refractive indices | Hardness | Cleavage | Density | 20.00 g piece in water |
|---|---|---|---|---|---|---|---|
| Quartz, Handbook sheet | SiO2 | Uniaxial (+) | ω 1.544, ε 1.553 | 7 | Rarely observable | 2.65 | 12.45 g |
| Danburite, Handbook sheet | CaB2Si2O8 | Biaxial (−) | α 1.627 to γ 1.639 | 7 to 7.25 | Indistinct on {001} | 2.93 to 3.02 | 13.17 to 13.38 g |
| Danburite, IGS | - | Biaxial (−); optically (+) at lower wavelengths | α 1.627 to γ 1.641 | 7 | Poor | 2.97 to 3.03, usually 3.00 | 13.27 to 13.40 g |
| Topaz, Handbook sheet | Al2SiO4F2 | Biaxial (+) | α 1.606 to γ 1.644 | 8 | Perfect on {001} | 3.49 to 3.57 | 14.27 to 14.40 g |
| Topaz, GIA | - | Not stated | 1.619 to 1.627 | 8 | Not stated | 3.53 | 14.33 g |
| Pekovite, Handbook sheet | SrB2Si2O8 | Biaxial (−) | α 1.597 to γ 1.632 | 6.5 to 7 | None | 3.35 | 14.03 g |
| Maleevite, Handbook sheet | BaB2Si2O8 | Biaxial (−) | α 1.649 to γ 1.656 | 7 | None | 3.78 | 14.71 g |
What is danburite? A calcium borosilicate
Danburite is calcium borosilicate, CaB2Si2O8, a grandfathered species first described in 1839 from the USA. That is the formula on the September 2026 IMA-CNMNC Master List; the Handbook of Mineralogy sheet writes the same atoms as CaB2(SiO4)2. On standard atomic weights the formula mass is 245.86 and the composition 22.81% CaO, 28.32% B2O3 and 48.88% SiO2, or 8.79% boron. The sheet’s one analysis, from Maglovec Hill, Prešov, Slovakia, prints 22.10, 27.65 and 49.18, within 0.71 of the ideal on every oxide; the sheet gives it as (Ca0.97Na0.01Mg0.01Fe0.01)(B1.95Al0.02)Si2.01O8.00.
What the sheet records. Orthorhombic, point group 2/m 2/m 2/m; crystals prismatic, rhomboidal to square in cross section, to 50 cm, and also disseminated masses. Cleavage indistinct on {001}; fracture uneven to subconchoidal; brittle. Hardness 7 to 7.25; density 2.93 to 3.02, calculated 2.99. Transparent to translucent; colourless, white, wine-yellow, yellowish brown or greenish; streak white; vitreous to greasy lustre. Biaxial (−), α 1.627 to 1.633, β 1.630 to 1.636, γ 1.633 to 1.639, 2V about 90°, with strong r < v dispersion.
Where it forms. The sheet’s Occurrence field gives granite and metamorphosed carbonate rocks associated with hydrothermal activity, and evaporites. Its associates include titanite, axinite, tourmaline, mica, quartz, grossular, albite, fluorite, apophyllite, stilbite, datolite, dolomite, calcite, anhydrite and gypsum.
Is danburite topaz? The middle-weight rule
The middle-weight rule: a 20.00 g piece of danburite reads 13.17 to 13.38 g in water, topaz 14.27 g or more and quartz 12.45 g, so danburite is the only one of the three in the 13 g band. Worked as 20 − 20/D on the sheets’ densities: danburite 2.93 to 3.02, topaz 3.49 to 3.57, quartz 2.65. The gaps are 0.89 g up to topaz and 0.72 g down to quartz, wide enough for a kitchen balance reading to 0.01 g on a clean piece. The International Gem Society’s danburite range, 2.97 to 3.03, reads 13.27 to 13.40 g and keeps 0.87 g clear of topaz. The method is on how to measure specific gravity at home.
Why the refractometer cannot do it, a documented failure. The danburite sheet’s indices run from α 1.627 to γ 1.639. The topaz sheet’s run from α 1.606 to γ 1.644 and contain them completely; GIA’s topaz facts give 1.619 to 1.627, meeting danburite’s lowest α exactly. Both sheets are orthorhombic and biaxial, and the International Gem Society calls danburite crystals prismatic with wedge-shaped terminations, like topaz. A cut stone can read the same on both counts.
Cleavage and hardness on rough. Topaz has one perfect cleavage on {001} and hardness 8; danburite’s cleavage on {001} is indistinct and its hardness 7 to 7.25. A flat, mirror-bright break across a prism points to topaz. Scratch tests damage specimens and are for rough only: how to test mineral hardness properly. The topaz page carries its own weight floor and fluorine check, which are not repeated here.
Danburite or quartz? Hardness 7 against 7
Danburite and quartz share hardness 7, so a scratch test cannot separate colourless danburite from rock crystal; weight and optics can. The quartz sheet prints 7, the danburite sheet 7 to 7.25. In water, 20.00 g of quartz reads 12.45 g and 20.00 g of danburite at least 13.17 g, 0.72 g heavier. That figure agrees with the quartz page, whose own identification rule is not repeated here.
Optics. Quartz is uniaxial (+), ω 1.544 and ε 1.553 on its sheet; danburite is biaxial (−) and starts at 1.627, 0.074 above quartz’s highest index. On a refractometer the two do not come near each other, and quartz gives two readings that never exceed 1.553.
Toughness, as the gem society puts it. The International Gem Society says danburite, with poor cleavage and hardness 7, ranks with quartz and topaz in toughness and wearability. That is the gem society’s judgement for jewellery, not a test; the sheet records danburite as brittle.
Pekovite and maleevite: the B2Si2O8 trio
Three IMA rows share the formula pattern B2Si2O8 and differ only in the large cation: danburite with calcium, pekovite with strontium and maleevite with barium. Method: the September 2026 list was converted with pdftotext -layout, each line parsed into name and formula (5,911 pairs), and every formula ending B2Si2O8 kept. Pekovite, SrB2Si2O8, is A, 2003-035, Tajikistan; maleevite, BaB2Si2O8, A, 2002-027, Tajikistan. Their sheets give both from the Dara-i-Pioz glacier, Alai range, Tien Shan.
How they compare. All three sheets are orthorhombic with point group 2/m 2/m 2/m and biaxial (−) optics. Pekovite reads density 3.35 and hardness 6.5 to 7; maleevite 3.78 and 7; both are white to colourless, with no cleavage. In water a 20.00 g piece reads 14.03 g as pekovite and 14.71 g as maleevite, 0.65 g and more above the heaviest danburite. The maleevite sheet prints bright blue fluorescence in shortwave UV; the danburite sheet prints none.
What this does not say. The sheets do not call the three a group, and neither does this page; the shared formula pattern is the finding. The danburite sheet records crystals to 50 cm, against grains to 2 mm for maleevite and 0.2 mm for pekovite.
Does danburite fluoresce? What the sheet prints
The Handbook danburite sheet prints no fluorescence, phosphorescence or luminescence, and this page states none. Method: the sheet text was searched for uoresc, uminesc and phosphoresc, a pattern that survives pdftotext rendering the fl ligature as a stray glyph; the same search finds the printed fluorescence on the hemimorphite and willemite sheets, the positive controls, and on the grossular and maleevite sheets. The lamp, the wavelengths and what fluorescence is are on fluorescent minerals explained.
The boron, by mass. The 8.79% boron worked above is the figure that makes danburite a borosilicate rather than a plain silicate; reedmergnerite, Na(BSi3O8), is the sodium borosilicate the sheet’s structure reference compares it with. No hazard is recorded on the sheet.
Colour. The sheet lists colourless, white, wine-yellow, yellowish brown and greenish. This page gives no cause for any of them.
Where danburite is found
The Distribution field names Danbury, Fairfield Co., Connecticut, and Russell, St. Lawrence Co., New York, USA; large crystals from Charcas, San Luis Potosí, and the La Verde mine near La Huerta, Baja California, Mexico; Alto Chapare, Cochabamba, Bolivia; Piz Valatscha, Mt. Scopi, south of Disentis, Graubünden, Switzerland; very large crystals from Anjanabonoina, and Maharitra on Mt. Bity, Imalo near Mania, and Sahasonjo in Madagascar; Alabashka near Mursinka in the Urals, large crystals from Vostochnaïa, Siberia, and Dal’negorsk, Primorskiy Kray, Russia; the Mogok district, Myanmar; and the Obira mine, Bungo, Oita Prefecture, and Toroku, Miyazaki Prefecture, Japan.
Cross-checked. The International Gem Society’s gem localities agree with the sheet at Danbury, Obira, Mt. Bity, Charcas and Mogok, and its Alto Chapare cat’s-eye cabochon matches the sheet’s Bolivian locality. Its other countries are not on the sheet and are not added here. No British locality is on the danburite sheet.
Related pages. Topaz and quartz, the two stones danburite is weighed against, are on topaz and quartz; the topaz-or-quartz question is on is topaz a quartz. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.