The Benitoite story: california's state gemstone
Benitoite (BaTiSi3O9)
The rolling, pine-studded hills of San Benito County, California, hide many rare and unusual minerals, but perhaps the most unusual of all is the gemstone called Benitoite.
Benitoite (Barium Titanium Silicate) is found in crystals of gem quality in only one place in the world—the small corner of San Benito County known as the New Idria District. Within the district, only one location --the Dallas Gem Mine-- had enough high-quality crystals to be commercially mined.
The crystals have no industrial uses due to their extreme rarity, but are highly prized in jewelry (cut gems can go for more than $1000 per carat), and as mineral specimens on the collector market.
Benitoite forms in hexagonal, flat pyramidal crystals, almost always deep sapphire blue in color, although occasional white to colorless or faintly pink specimens have been recovered. Their color and brilliance, as well as a hardness of 6- 6 ½ on the Mohs scale, makes them valuable for jewelry and faceting.
The crystal form is quite unique; benitoite is distinctive among minerals for being the first discovered representative of the ditrigonal dipyramidal crystal class.
Like so many other valuable mineral deposits, the benitoite location was discovered purely by chance. In February of 1907, a prospector named James Marshall Couch went to the oilfields near Coalinga, California, in the hope of finding a job. There he met with oil company superintendent R.W. Dallas, who, together with miner T.E. Sanders, agreed to lend Couch $25 each and sent him out to prospect for copper and cinnabar in the nearby Diablo Mountains.
On Couch’s third day in the mountains, he stumbled across a hillside near his campsite where the ground was “… littered in thousands of blue crystals, weathering out of the nearby natrolite veins.” Couch at first believed them to be “blue diamonds” or “sapphires”, such was their beauty and brilliant color.
Exploring his find, he pushed through some brush to uncover the entrance of a small cave, and stepped into what looked like a giant geode. Under his feet lay thousands or sapphire-blue crystals, and more were embedded in the white natrolite walls of the cave.
Overcome with excitement about his find, Couch gathered up a pocketful of the gems and rushed back to town to show his find to Dallas, who prudently sent him straight back to stake a claim on the location.
With the claim safely staked, then came the task of identifying just what they had found. Specimens were sent to a lapidary “expert” in Los Angeles, who dismissed them as merely being “blue obsidian." Unsatisfied with this answer, Dallas had samples sent to the University of California, Berkeley for identification. After analysis by Dr. George Louderback, it was found to be, not diamonds, not sapphires, and most certainly not obsidian, but instead an entirely new mineral.
Benitoite is usually found in a matrix of three other minerals—natrolite, hard blue schist, and cement-like crossite. In the early days of mining at the Dallas Mine, the hardness of the matrix was a great source of frustration to the miners. Eventually, in an effort to easily remove the gems, the miners dynamited the main vein, utterly obliterating the geode cave that Couch had stepped into.
The remaining benitoite was then extracted by using a punch-press on the large boulders that contained the crystals, grinding both the rocks and most of the crystals inside to worthless shards. It has been estimated that only one out of a hundred gem crystals survived this method intact, while the other 99% were pulverized into fine blue dust and blown away on the hot Diablo Range winds.
Many years later, miners discovered that muriatic acid easily eats away the natrolite and schist of the matrix and exposes the benitoite without damaging the crystals. By then, of course, countless numbers of irreplaceable crystals had already been lost.
For many years after its initial discovery, benitoite was known to occur only in a limited area of the New Idria district in San Benito County, California. However, very small crystals have since been identified at four other locations in California, two of which are in neighboring Fresno County, one in Kern County, and one on Trumbull Peak in Mariposa County. Traces of the mineral have also been found at Magnet Cove, Arkansas, in Niigata Prefecture, Japan and at Broken Hill, New South Wales, Australia.
However, all of these occurrences have yielded only minute grains of benitoite, not usable gems. Only very spotty records exist on the production of the mine, especially in the early days, but the early mining logs speak of taking gem crystals out “by the sackful” every week— each individual sack could probably have contained an excess of 1000 carats, but the exact amounts went unrecorded. Between the date of the benitoite discovery in 1907 and 2003, it is estimated that only 10,000 carats of the gem were cut and polished.
In 2002 alone, the mine produced more than 1,500 carats. In 2003, it was estimated that the mine could produce 2,500 carats per year, or 25% of the previous of the previous 90 year’s output, but the mine only stayed commercially open for two more years.
It is doubtful that there are any significant veins remaining to be discovered at the location; as far as commercial extraction goes the deposit appears to be completely mined out. Perhaps one day a new deposit of benitoite will be discovered; until then, the material already taken out of the San Benito hills is all we have.
Benitoite (Barium Titanium Silicate) is found in crystals of gem quality in only one place in the world—the small corner of San Benito County known as the New Idria District. Within the district, only one location --the Dallas Gem Mine-- had enough high-quality crystals to be commercially mined.
The crystals have no industrial uses due to their extreme rarity, but are highly prized in jewelry (cut gems can go for more than $1000 per carat), and as mineral specimens on the collector market.
Benitoite forms in hexagonal, flat pyramidal crystals, almost always deep sapphire blue in color, although occasional white to colorless or faintly pink specimens have been recovered. Their color and brilliance, as well as a hardness of 6- 6 ½ on the Mohs scale, makes them valuable for jewelry and faceting.
The crystal form is quite unique; benitoite is distinctive among minerals for being the first discovered representative of the ditrigonal dipyramidal crystal class.
Like so many other valuable mineral deposits, the benitoite location was discovered purely by chance. In February of 1907, a prospector named James Marshall Couch went to the oilfields near Coalinga, California, in the hope of finding a job. There he met with oil company superintendent R.W. Dallas, who, together with miner T.E. Sanders, agreed to lend Couch $25 each and sent him out to prospect for copper and cinnabar in the nearby Diablo Mountains.
On Couch’s third day in the mountains, he stumbled across a hillside near his campsite where the ground was “… littered in thousands of blue crystals, weathering out of the nearby natrolite veins.” Couch at first believed them to be “blue diamonds” or “sapphires”, such was their beauty and brilliant color.
Exploring his find, he pushed through some brush to uncover the entrance of a small cave, and stepped into what looked like a giant geode. Under his feet lay thousands or sapphire-blue crystals, and more were embedded in the white natrolite walls of the cave.
Overcome with excitement about his find, Couch gathered up a pocketful of the gems and rushed back to town to show his find to Dallas, who prudently sent him straight back to stake a claim on the location.
With the claim safely staked, then came the task of identifying just what they had found. Specimens were sent to a lapidary “expert” in Los Angeles, who dismissed them as merely being “blue obsidian." Unsatisfied with this answer, Dallas had samples sent to the University of California, Berkeley for identification. After analysis by Dr. George Louderback, it was found to be, not diamonds, not sapphires, and most certainly not obsidian, but instead an entirely new mineral.
Benitoite is usually found in a matrix of three other minerals—natrolite, hard blue schist, and cement-like crossite. In the early days of mining at the Dallas Mine, the hardness of the matrix was a great source of frustration to the miners. Eventually, in an effort to easily remove the gems, the miners dynamited the main vein, utterly obliterating the geode cave that Couch had stepped into.
The remaining benitoite was then extracted by using a punch-press on the large boulders that contained the crystals, grinding both the rocks and most of the crystals inside to worthless shards. It has been estimated that only one out of a hundred gem crystals survived this method intact, while the other 99% were pulverized into fine blue dust and blown away on the hot Diablo Range winds.
Many years later, miners discovered that muriatic acid easily eats away the natrolite and schist of the matrix and exposes the benitoite without damaging the crystals. By then, of course, countless numbers of irreplaceable crystals had already been lost.
For many years after its initial discovery, benitoite was known to occur only in a limited area of the New Idria district in San Benito County, California. However, very small crystals have since been identified at four other locations in California, two of which are in neighboring Fresno County, one in Kern County, and one on Trumbull Peak in Mariposa County. Traces of the mineral have also been found at Magnet Cove, Arkansas, in Niigata Prefecture, Japan and at Broken Hill, New South Wales, Australia.
However, all of these occurrences have yielded only minute grains of benitoite, not usable gems. Only very spotty records exist on the production of the mine, especially in the early days, but the early mining logs speak of taking gem crystals out “by the sackful” every week— each individual sack could probably have contained an excess of 1000 carats, but the exact amounts went unrecorded. Between the date of the benitoite discovery in 1907 and 2003, it is estimated that only 10,000 carats of the gem were cut and polished.
In 2002 alone, the mine produced more than 1,500 carats. In 2003, it was estimated that the mine could produce 2,500 carats per year, or 25% of the previous of the previous 90 year’s output, but the mine only stayed commercially open for two more years.
It is doubtful that there are any significant veins remaining to be discovered at the location; as far as commercial extraction goes the deposit appears to be completely mined out. Perhaps one day a new deposit of benitoite will be discovered; until then, the material already taken out of the San Benito hills is all we have.
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