Benitoite: Gemstone Information

The original paper describing the discovery of benitoite was the work of Louderback; Benitoite, a new California gem mineral, Univ. Calif., Bull. Dept.Geol. 5, 149–53, 1907. W. C. Blasdale provided a chemical analysis. The same author later published, in the same series, 5, 331–380, 1909; Benitoite,its paragenesis and mode of occurrence. In a paper by Laird and Albee (American Mineralogist 57, 85–102, 1972), it is reported that benitoite may be milky white, colourless or blue but that the composition does not vary with colour. The blue is believed by Nassau (Physics and Chemistry of  Colour, second edition, 1983) to arise from intervalence change transfer between iron and titanium (as with blue sapphire). It is likely that the blue of tanzanite and of iolite arises from the same process. Excellent summaries of the sources of benitoite can be found in editions of Gemstones of North America (Sinkankas), several accounts of the early twentieth century disagree with one another over the first benitoite location to be found.It seems fairly certain that during the autumn of 1906, a prospector searching for mercury and copper minerals in the area of the headwaters of the San Benito river of California discovered blue crystals in a vein of white natrolite. They were subsequently considered to be sapphire. The strong dichroism shown by the stones, however, led to further investigation in the course of which specimens were sent to Louderback, of the University of California, who identified them as a new mineral to which the name benitoite was given, after the locality in which they were discovered. Kunz, on the other hand, states [in Gems and Precious Stones of North America 1890 that the crystals were found in 1907 by Hawkins and Edwin Sanders who were prospecting in the southern part of the Mount Diablo range near the San Benito-Fresno border, and that these crystals were brought to the attention of Dr Louderback by Shreve and Company, a San Francisco firm who had purchased one of the cut stones from a lapidary and who were later offered some of the rough material as sapphire.Benitoite is a fine bright near sapphire-blue and crystallizes uniquely in a class of the hexagonal system which has a trigonal axis of symmetry and a plane of symmetry at right angles to it, a form that was only postulated in nature but not found until the discovery of benitoite. Colourless crystals of benitoite are sometimes cut, perhaps for their occasionally orange fluorescence.Benitoite is barium titanium silicate with the formula BaTiSi3O9. The SG lies between 3.65 and 3.68, but the latter value is more common. The hardness is about 6.5 on Mohs’ scale. The RIs are 1.757 for the ordinary ray and 1.804 for the extraordinary ray, uniaxial positive.The birefringence is strong at 0.047. The pronounced dichroism shows twin colours of blue for the extraordinary ray and colourless for the ordinary ray, so that to obtain the best colour the table facet should be cut parallel to the principal crystal axis,and as most of the crystals have a tabular habit this precludes the cutting of large stones. Benitoite has a high colour dispersion, 0.039 for the ordinary ray and 0.046 for the extraordinary ray (compare the 0.044 of diamond).The lower density allows distinction from sapphire, but if the stone isset determination of this constant would not be possible and a refrac-tometer reading would be necessary. Care is needed, however, in observing the shadow edges as the maximum value of the extraordinary ray is beyond the range of the refractometer and may be missed unless the stone is rotated fully. The pronounced dichroism may also distinguish it.Benitoite shows a bright blue fluorescence under SWUV but is inert to LW. Some colourless specimens have been found to give a reddishorange glow under LW.At the time of writing, gem benitoite still comes only from mines in San Benito County, California, USA, where it occurs with joaquinite andneptunite in natrolite veins cutting glaucophane schist.