Howlite
Howlite is Ca2SiB5O9(OH)5, a white calcium borosilicate of borate deposits: hardness 3.5, density 2.53 to 2.59, 13.81% boron by mass, nodular, often dyed blue.
Howlite is a calcium borosilicate, Ca2SiB5O9(OH)5 on the September 2026 IMA list, found in borate deposits. Its Handbook sheet gives monoclinic crystals to 1 cm, most commonly nodules, white, hardness 3.5 and density 2.53 to 2.59. Worked from the formula it is 13.81% boron by mass. Dyed blue, it imitates turquoise (International Gem Society).
In short
- One composition, two orders. The September 2026 IMA-CNMNC Master List lists howlite as Ca2SiB5O9(OH)5, status G, 1868, country Canada. The Handbook of Mineralogy howlite sheet writes Ca2B5SiO9(OH)5. Both have 2 calcium, 5 boron, 1 silicon, 9 oxygen and 5 hydroxyl.
- The boron ledger. On standard atomic weights the formula mass is 391.32 and boron is 13.81% of it; the oxide ideal, B2O3 44.48, SiO2 15.35, CaO 28.66, H2O 11.51, is what the sheet prints. Its Windsor, Nova Scotia, analysis gives 13.83% boron and its Daggett, California, analysis 13.78%. Colemanite is 15.78% boron and ulexite 13.34%; turquoise and magnesite contain none.
- Light for a white stone. A 20.00 g piece weighs 12.09 to 12.28 g in water at the sheet’s 2.53 to 2.59, against 11.75 g for colemanite and 9.77 g for ulexite, its associates in borate deposits, and 13.33 g for magnesite.
- The sources disagree on UV. The howlite sheet records no fluorescence. The International Gem Society howlite article records brownish yellow under shortwave and, in some California material, deep orange under longwave. Both are printed here; neither is a test.
- White by nature, blue by dye. The sheet’s colour is white. GIA lists howlite among materials dyed since ancient times, and the gem society says it is frequently dyed blue to resemble turquoise and makes a most convincing simulant. That question is answered on is howlite turquoise.
| Species | IMA formula | Hardness | Density | 20.00 g piece in water | Boron by mass | UV on the sheet |
|---|---|---|---|---|---|---|
| Howlite | Ca2SiB5O9(OH)5 | 3.5 | 2.53–2.59 | 12.09–12.28 g | 13.81% | Nothing recorded |
| Colemanite | CaB3O4(OH)3(H2O) | 4.5 | 2.423 | 11.75 g | 15.78% | Bright pale yellow; may phosphoresce pale green |
| Ulexite | NaCaB5O6(OH)6(H2O)5 | 2.5 | 1.955 | 9.77 g | 13.34% | May fluoresce yellow, greenish yellow, cream, white, SW and LW |
| Magnesite | Mg(CO3) | 3.5–4.5 | 3.00 | 13.33 g | 0 | May show pale green to pale blue; triboluminescent |
| Turquoise | CuAl6(PO4)4(OH)8(H2O)4 | 5–6 | 2.86, less if massive | 13.01 g | 0 | Nothing recorded |
What is howlite? The IMA row and the sheet
The row. The September 2026 IMA-CNMNC Master List gives Ca2SiB5O9(OH)5, status G for grandfathered, year 1868, country Canada, with the first reference A System of Mineralogy, 5th edition (Wiley, New York, 1868) and the second American Mineralogist 73 (1988), 1138. The country column records where a mineral was first described; it is not a type-locality field. The sheet’s Name field says Henry How of King’s College, Windsor, Nova Scotia, first described the species.
On the sheet. Monoclinic. Crystals tabular and flattened, to 1 cm; most commonly nodular; chalky to porcelaneous when massive. For porcelaneous types the fracture is nearly even and smooth. Hardness 3.5; density 2.53 to 2.59 measured, 2.61 calculated. Translucent in thin fragments; white, colourless in thin section; lustre subvitreous, glimmering. Biaxial (−), α = 1.583 to 1.586, β = 1.596 to 1.598, γ = 1.605. The sheet prints no streak and no cleavage, and this page adds neither.
The gem society’s panel. The International Gem Society gives hardness 3 to 3.5, specific gravity 2.45 to 2.58, the same refractive indices, no cleavage and no pleochroism, and says howlite is always opaque in nodules and sometimes contains black, threadlike impurities resembling the veining in turquoise.
How much boron is in howlite? The boron ledger
Worked. Ca2SiB5O9(OH)5 has a formula mass of 391.32 on standard atomic weights. Its five boron atoms, 10.81 each, are 13.81% of the mass; as oxides, B2O3 44.48, CaO 28.66, SiO2 15.35, H2O 11.51, the ideal column the sheet prints. Howlite is about one seventh boron by weight, and its silicon is a single atom against five of boron.
Against the analyses. The sheet’s Windsor, Canada, analysis has B2O3 44.52, which is 13.83% boron; it was recalculated to 100% after deducting 4.32% gypsum. The Daggett, California, analysis has B2O3 44.38, 13.78%. Both sit within 0.03 percentage points of the ideal.
Against its associates. The sheet names colemanite, ulexite and bakerite as associates. Colemanite, Ca2B6O11·5H2O on its sheet, is 15.78% boron; ulexite, NaCaB5O6(OH)6·5H2O, 13.34%. Bakerite has no row on the September 2026 IMA list by the same script, so it is not tabulated. Ulexite’s own page is ulexite.
Howlite, colemanite or ulexite? Weight in water
Three white borates, three readings. Worked as 20 − 20/D for a 20.00 g piece: howlite 12.09 to 12.28 g, colemanite 11.75 g, ulexite 9.77 g. Howlite and colemanite are 0.34 g apart at the nearest figures; ulexite is more than 2 g lighter than either. The gem society’s howlite range, 2.45 to 2.58, reads 11.84 to 12.25 g and closes the gap to colemanite to 0.09 g, so weight alone does not separate those two. The method, and the porosity caveat that matters on nodular material, are on how to measure specific gravity at home.
What else separates them on the sheets. Colemanite is 4.5 hard with a perfect cleavage and forms crystals to 30 cm; ulexite is 2.5 hard and typically fibrous, its parallel fibres able to act as light pipes; howlite is 3.5, without a printed cleavage, and usually nodular. Colemanite and ulexite both record fluorescence on their sheets; howlite’s records none.
Is howlite fluorescent? Two sources, two answers
The Handbook sheet records no luminescence; the International Gem Society records some. Read ligature-aware, with hemimorphite and willemite as positive controls, the howlite sheet has no fluorescence entry. The gem society’s panel reads Brownish yellow in SW; some California material deep orange in LW.
How to read that. A sheet that records nothing has not said a mineral is inert, so the two are not in direct conflict; the gem society’s entry is the only positive statement in the sources fetched. Neither source gives a cause, and this page supplies none. Because dyes and fillers can respond to a lamp themselves, this page does not offer UV as a test for dyed howlite; see how to tell if a crystal is dyed.
Where howlite is found, and why it is often blue
The sheet’s Distribution field. In Canada, near Windsor and at Wentworth, with large crystals on the shore of Bras D’Or Lake, 2.5 km south of Iona, Cape Breton Island, Nova Scotia. In California, the Sterling Borax mine, Tick Canyon, near Lang, Los Angeles County; near Calico, San Bernardino County; Gower Gulch and the Billie mine near Ryan, Inyo County; the Russell and other borate mines north of Lockwood Valley, Ventura County; and the Kramer deposit, Kern County. Also Magdalena, Sonora, Mexico; Susurluk, Turkey; and Baljevac na Ibru, near Kraljevo, and near Bela Stena, Yugoslavia, as the sheet names it.
Blue howlite is dyed howlite. The sheet gives white as its only colour. The GIA gem treatment page names howlite with turquoise, lapis lazuli and coral among materials dyed since ancient times, and says many dyes can be removed by solvents such as alcohol or acetone. Separating dyed howlite from turquoise is on is howlite turquoise, and from dyed magnesite on is magnesite the same as howlite, whose same-volume example is not repeated here.
On this site. The species pages are listed in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.