LET'S TALK GEMSTONES


BENITOITE

By Edna B. Anthony Gemologist

One of the most beautiful blue gemstones is native to our own country. In 1906, a prospector found what he thought were sapphire crystals in the Diablo Mountain range of San Benito County, California. A year later, mineralogist, G. D. Louderback, identified them as a new mineral which he named benitoite. The Dallas mine, there, is still-- the only known source of gem quality rough of this "heavenly stone." The original deposit has been depleted, although some material is still being recovered and sold. Even the largest of the heavily flawed crystals yield only small areas suitable for faceting. 0.5-carat fragments are common at the mine, but material larger than two to three carats is not readily available. The flattened triangular crystals are found in a matrix of crossite (a magnesiun amphibole) and natrolite in veins of altered serpentinites. Neptunite and an even rarer mineral, joaquinite, are associated with the benitoite. A Japanese area in the Niigata prefecture near Omi Machi, Nishi Kubiki Gun is a source of six sided benitoite crystals. Small grains are also found in Belgium and in Texas.

The largest known faceted benitoite weighs 15.42 carats. In his COLOR ENCYCLOPEDIA OF GEMSTONES, Dr. Joel Arem states that a 6.52-carat gem was stolen in transit after being cut for a private collector. it has not been recovered. 0.5-carat stones are not uncommon, but those over one carat are rare. Despite its beauty, benitoite is not often seen in jewelry. The lack of an abundant supply, a hardness just above that of opal and only fair to poor toughness limit its use. It exhibits no well- developed cleavage since it is a 3-member ring silicate, but weak atomic bonds make it fragile. Benitoite's strong pleochroism influences the cutting. An almost colorless crystal can exhibit, along the main crystal axis, a rich blue color that rivals that of fine sapphires. its high lustre and a dispersion often exceeding that of diamond, make it a lovely choice for well designed jewelry to be worn with care.

Benitoite does occur in violet tints. Richard T. Liddicoat, in his HANDBOOK OF GEM IDENTIFICATION, says these tints are due to strong dispersion. He notes also that a small pink benitoite has been found and identified by a GIA graduate. This may be the 0.5-carat pink stone referred to by John Sinkankas in the second edition of STANDARD CATALOGUE OF GEM VALUES by Anna Miller and John Sinkankas. The textbook, GEMOLOGY FOR THE JEWELER by Orlando Paddock and Malcolm Heuser states, "a few pink stones have been found," and the course material for gemology at Paris, Texas Junior College indicates that benitoite "may show color zoning."

Since benitoite is an uncommon mineral, a list of some photographic material presented in books available in a good library, may be helpful. Gemological information will follow.
Page 40 -- THE COLOR TREASURY OF CRYSTALS -translated from the Italian by Vicenzo De Michele photos by Carlo Bevilacqua published by Crescent - a photo of a magnificent 10 mm crystal in matrix on exhibit at the Museum of Natural listory. Milan, Italy.

Page 270 -- MINERALS AND ROCKS by, Dr Jiri Kourimsky - photos by F. Tvrz published by Chartwell Books, Inc. - three beautiful crystals (14 mm) in matrix.

Page 50 -- AN ILLUSTRATED GUIDE T O ROCKS AND MINERALS by Michael O'Donoghue - published by Smithmark Publishers, Inc. several excellent crystals in matrix.

Page 76 -- AMERICAN NATURE GUIDES TO ROCKS AND MINERALS by Michael O'Donoghue - published by Gallery Books - photo of cushion cut gem and crystal in matrix.

Page 185 -- GEMSTONES OF THE WORLD by Walter Schumann published by Sterling Publishing Company, Inc. -crystals in matrix and eight cut gems.

Page 235 -- HANDBOOK OF GEM IDENTIFICATION by Richard T. Liddicoat - published by GIA - photo showing dispersion in benitoite.

Page 416 -- PHOTO ATLAS OF INCLUSIONS IN GEMSTONES by E. J. Gubelin and J. I. Koivula published by ABC Edition, Zurich three photos of inclusions in benitoite, including "a very rare two-phase liquid and gas inclusion" with "a secondary fingerprint can also be seen highlighting the larger fluid inclusion."

Specie: BENITOITE

Composition: BaTi(Si30,) - Barium Titanium Silicate
Varieties: Transparent (by color), Translucent
Colors: Colorless, White, Blue (light to dark), Violet, Pink (very rare)
Phenomena: None Known
Streak: White
Crystal System: Trigonal
Habit: Bipyramidal
Cleavage: None to Poor
Lustre: Vitreous
Fracture: Conchoidal--Brittle
Fracture Lustre: Vitreous
Specific Gravity: 3.64-3.67
Hardness (Mho's): 6-6.5
Toughness: Fair
Refractive Index: 1.757-1.804
Birefringence: 0.047
Optic Character: Uniaxial-Positive
Dispersion: 0.039-0.047
Pleochroism: Strong-Blue to Green/Colorlesss in Blue Stones, Purple/Reddish in Violet Stones
Ultraviolet Fluorescence: SW-Strong Blue, LW-Inert
Spectra: Not Diagnostic
Color Filter: No Information
Solubility: May be sensitive to borron compounds
Inclusions: Natrolite, Crossite, Two-Phase Liquid and Gas (Rare)
Treatments: None
Thermal Traits: Avoid Thermal Shock Remove Stone for Jewelry Repairs

See also: Cutting Benitoite