Benitoite
Benitoite is BaTiSi3O9: hexagonal, hardness 6 to 6.5, density 3.65, blue under shortwave UV. Its sheet names ten places, three of them outside California.
Benitoite is a barium titanium silicate, BaTiSi3O9: hexagonal, hardness 6 to 6.5, density 3.65, sapphire-blue to colourless, with blue fluorescence under shortwave UV. It is grandfathered on the IMA list, 1907, USA, and named for San Benito County, California. It is not found only in California: its Handbook sheet names Arkansas, Japan and Australia too.
In short
- One formula, three rows. The September 2026 IMA-CNMNC Master List gives benitoite as BaTiSi3O9, status G, 1907, USA. A script over the list finds three rows written Ba + one element + Si3O9: benitoite (Ti), bazirite (Zr) and pabstite (Sn). On standard atomic weights benitoite is 33.22% barium and 11.58% titanium by mass.
- Not only California. The Handbook of Mineralogy sheet’s Distribution field names ten places: five in San Benito County, two elsewhere in California (Big Creek–Rush Creek, Fresno Co.; Trumbull Peak, Mariposa Co.), and three outside it: the Diamond Jo quarry, Magnet Cove, Arkansas; Ohmi, Niigata, Japan; and Broken Hill, New South Wales. Gems & Gemology (Laurs et al., 1997) says commercial gem quality is known from one mine, the Benitoite Gem mine.
- The 0.04 line. Benitoite’s indices on the sheet are 0.046 to 0.047 apart, worked. Of the eight gems Laurs et al. list as possible confusions, only zircon also clears 0.04 on its sheet, and zircon reads 15.65 g or more in water for a 20.00 g piece against benitoite’s 14.52 to 14.74 g. Sapphire, the confusion of 1907, is 0.008 to 0.009.
- Blue under shortwave. The sheet: Blue fluorescence under SW UV. Laurs et al. found blue stones inert under longwave and colourless ones dull red. The detail, and how a Gem-mine plate looks under the lamp, is on is benitoite fluorescent.
- State gemstone since 1985. The California State Capitol Museum gives the designation as Assembly Bill 2357, Chapter 1365, 1985. Laurs et al. note the gem crystals grew in natrolite veins and are usually coated by it, which is why the white mineral on so many specimens is natrolite.
| Gem (species) | Hardness | Density | 20.00 g piece in water | Optic class | Birefringence, worked | Clears 0.04? |
|---|---|---|---|---|---|---|
| Benitoite | 6–6.5 | 3.65 (Laurs: 3.65–3.80) | 14.52 g (to 14.74 g) | Uniaxial (+) | 0.046–0.047 (Laurs measured 0.043–0.047) | Yes |
| Sapphire (corundum) | 9 | 3.98–4.10 | 14.97–15.12 g | Uniaxial (−) | 0.008–0.009 | No |
| Tanzanite (zoisite) | 6–7 | 3.15–3.36 | 13.65–14.05 g | Biaxial (+) | 0.012–0.018 | No |
| Iolite (cordierite) | 7–7.5 | 2.60–2.66 | 12.31–12.48 g | Biaxial (+) or (−) | 0.010–0.018 | No |
| Blue spinel | 7.5–8 | 3.6–4.1 | 14.44–15.12 g | Isotropic | 0 | No |
| Sapphirine | 7.5 | 3.40–3.58 | 14.12–14.41 g | Biaxial (+) or (−) | 0.004–0.008 | No |
| Blue tourmaline (elbaite) | 7 | 2.90–3.10 | 13.10–13.55 g | Uniaxial (−) | 0.018–0.021 | No |
| Blue diamond | 10 | 3.511 | 14.30 g | Isotropic | 0 | No |
| Blue zircon | 7.5 | 4.6–4.7 | 15.65–15.74 g | Uniaxial (+) | 0.054–0.055 | Yes: weight separates it |
What is benitoite? BaTiSi3O9, as the IMA list and the sheet give it
The names. The September 2026 IMA-CNMNC Master List lists benitoite as BaTiSi3O9, status G, grandfathered, 1907, with the USA as the country of first discovery; the Handbook of Mineralogy benitoite sheet gives the same formula. Its Name field reads for the occurrence in San Benito Co., California, USA, and its first reference is Louderback (1907), Benitoite, a new California gem mineral. Type material: n.d. Laurs et al. (Gems & Gemology, 1997) add that the name also follows the San Benito River and San Benito Mountain.
What it looks like, per the sheet. Hexagonal; flat pyramidal crystals, tabular on {0001}, triangular or hexagonal in outline, to 6 cm, twinned by rotation about [0001], forms read from the rendered sheet. Cleavage poor on {10-11}, fracture conchoidal, hardness 6 to 6.5. Colour sapphire-blue, white to colourless or pink, commonly bicoloured, vitreous. Uniaxial (+), with O colourless and E purple, indigo or greenish blue: the strong dichroism Laurs et al. saw through a dichroscope in every direction.
The analysis, worked. The sheet’s first San Benito analysis, SiO2 43.68, TiO2 20.09, BaO 36.33, gives 0.972 barium, 1.032 titanium and 2.982 silicon per nine oxygens: the formula, within a few hundredths. The barium figure, 33.22% by mass in the ideal formula, matches the one on is baryte toxic.
Is benitoite only found in California? What the sheet’s Distribution field lists
No, on the sheet’s own list. The Distribution field reads, in full: in California, at the Gem mine, Mina Numero Uno, the Victor and Junilla claims, and on Santa Rita Peak, San Benito Co.; from Big Creek–Rush Creek, Fresno Co.; on Trumbull Peak, near Incline, Mariposa Co.; in the Diamond Jo quarry, Magnet Cove, Hot Spring Co., Arkansas; from Ohmi, Niigata Prefecture, Japan; and at Broken Hill, New South Wales, Australia. That is ten named places, seven of them in California.
Where the gems come from is a narrower question. Laurs et al. (1997) say benitoite had then been confirmed from nine locations, that only the Benitoite Gem mine and the Junnila claim had produced gem-quality material, and that all commercial gem production came from the Gem mine. Their nine omit Trumbull Peak, which the sheet includes; they also record detrital grains in Kern County that are not in place. The district’s history is on the Benitoite Gem Mine; the Australian locality is Broken Hill.
Its company. The sheet’s Occurrence field: in natrolite veins cutting glaucophane schist in a serpentine body (San Benito Co.), and in a magnesio-riebeckite–quartz–phlogopite–albite dike cutting serpentinite (Ohmi). Associates: neptunite, joaquinite and natrolite in California; ohmilite, bario-orthojoaquinite and leucosphenite at Ohmi. The neptunite sheet gives black prisms to 7.5 cm, hardness 5 to 6, with a cinnamon-brown streak.
Benitoite or sapphire? The 0.04 line
The confusion is the founding one. Laurs et al. record that the first lapidary to cut the material called it spinel because it was too soft for a sapphire, and that a jeweller found it doubly refractive with a dichroscope and sent it to George Louderback at Berkeley. Their table lists eight gems benitoite may be confused with: sapphire, tanzanite, blue diamond, iolite, blue tourmaline, blue spinel, sapphirine and blue zircon. GIA names the species behind three of those trade names: sapphire is corundum, tanzanite is zoisite, iolite is cordierite.
The rule, worked from the sheets. Subtract each sheet’s low refractive index from its high one at matching ends of the range. Benitoite gives 0.046 to 0.047; Laurs measured 0.043 to 0.047 on 21 stones. Of the eight look-alikes in the table above, only zircon also clears 0.04 (0.054 to 0.055); the rest run from 0 for spinel and diamond to 0.021 for elbaite. Laurs et al. say the doubling of pavilion facet junctions is easily seen through the table at 10×. Doubling that strong, in a blue stone, leaves benitoite or zircon.
Then weigh it. A 20.00 g piece reads 14.52 to 14.74 g in water as benitoite and 15.65 to 15.74 g as zircon, so the scale finishes what the loupe starts; the method is on how to measure specific gravity at home. Documented failure: Laurs et al. measured cabochons with abundant fibrous inclusions as low as 3.31, which reads 13.96 g, inside tanzanite’s range; the line holds for clean material only. Zircon’s own metamict exception is on zircon, and corundum has its own page.
Bazirite and pabstite: the two other BaMSi3O9 rows
Method and count. Every line of the September 2026 IMA list, saved as text with pdftotext -layout, was parsed by script into name and formula (controls: the benitoite, mimetite, natrolite and neptunite rows must parse with their known formulae), and an independent raw scan of every line checked the result. Three rows are written exactly Ba + one element + Si3O9: benitoite (Ti, G, 1907, USA), bazirite (Zr, A, 1976-053, United Kingdom) and pabstite (Sn, A, 1964-022, USA). Eight rows end in Si3O9 in all, including wadeite, K2ZrSi3O9.
What the sheets add. The bazirite and pabstite sheets give both as hexagonal, like benitoite, and both colourless or nearly so, pabstite with a pink tinge: bazirite’s X-ray pattern is from Rockall Island, Scotland, and pabstite’s from Santa Cruz, California. Laurs et al. found tin and zirconium the most variable trace elements in benitoite and tie that to these two analogues.
The point for a label. A colourless barium silicate from a British or Californian pegmatite is not benitoite by default; the formula family has three members, and only one is a gem. 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.