Benitoite Gem Mine, San Benito County, California
INTRODUCTION:Benitoite is one of the world's rarest gemstones and possesses many uni
Benitoite has been confirmed from several locations around the world, but gem quality crystals have only been found at the historic Dallas claim (popularly known as the “Benitoite Gem mine”) and the nearby Junnila claim, both located in the New Idria district, San Benito County, California. In 1985, the State of California officially named benitoite the State Gemstone.
The following project review provides historical information concerning the benitoite gem occurrence at the Dallas Claim and the efforts of Collector’s Edge Minerals, Inc. (CEMI) to conduct specimen mining and gemstone recovery during the period 2000 through 2004. CEMI operated the Benitoite Gem mine under the name of Benitoite Mining Inc. until 2004 and subsequently sold the property after their period of active mining.
LOCATION:The Benitoite Gem mine is located on a small mining claim (the Dallas claim) in San Benito County, California, about halfway between Los Angeles and San Francisco. Access to the mine is via either a roughly sealed trail that is privately controlled or by two alternate dirt roads that approach from the west and north of the gem mine. General approaches to the local roads is either to the northwest from Coalinga, southeast from Paicines, or northeast from King City. This area of the coastal range is sparsely populated and there are no facilities within miles of the Benitoite Gem mine.
Map provided with permission of J. Veevaert, Trinity Minerals, USA.
The Benitoite Gem mine is located on 16.2 hectares of private patented mining property. Four other benitoite prospects in the district are claimed as follows: (1) The Junnila claim; (2) The Mina Numero Uno claim; (3) The Victor claim; and (4) The Santa Rita Peak property. All of these mineral prospects are closed to the public. (Laurs et al., 1997)
This simplified geologic map of the New Idria district shows the distribution of benitoite deposits. Benitoite is found within Franciscan blueschist bodies that have been tectonically incorporated into the serpentinite (after Eckel and Myers, 1946; Coleman, 1957, Dibblee, 1979; and Laurs, 1995). This map is found in a Gems & Gemology article, Vol. 33, No. 3, p.170 written by Brendan M. Laurs, et al., 1997.
The Benitoite Gem mine is famous for its rare and unique gemstone, benitoite.
A prospector, James Couch, was grubstaked by Roderick Dallas, and in February, 1907, and on his way to investigate some intriguing outcrops, found a small area littered with blue crystals which he thought might be blue sapphires. He collected several and rushed back to Coalinga. A claim was placed which was named the Dallas Gem mine. Dr. George Louderback, a professor of mineralogy at the University of California, Berkeley, was provided some of the stones. Dr. Louderback soon realized that they were not sapphires or spinel as some thought, but a new mineral not known to science. In July 1907 he published an article, naming the new mineral benitoite, named after San Benito County. The black mineral associated with the benitoite was initially called ‘carlosite’, named after the nearby San Carlos peak. However, he later discovered that this mineral was neptunite which had been discovered in Greenland in 1893.
When word got out of the new discovery, several people, including Dr. George Kunz from Tiffany's of New York, rushed to the site to secure an exclusive marketing agreement with the miners. Mr. G. Eacret, of Shreve and Company in San Francisco won the marketing rights.
The mine owner, Mr. R.W. Dallas, built a mine camp and immediately expanded mining operations. The mine produced benitoite from an open cut in the hillside, as well as a short underground tunnel pushed into an outcrop of benitoite-bearing material called blueschist. For about 5 years, the blueschist layer yielded thousands of excellent gemstones.
Stock certificate for 10 shares in the Dallas Mining Company, dated February, 1907.
Roderic Dallas (left), first owner of the mine and James Couch (right), discoverer of the deposit, circa 1907.
Cabin built at the Dallas Gem mine in 1907 (James Couch, second from left).
Circa 1910, miners in front of the main portal to the Benitoite Gem mine. Photo from the Dallas Mining Company archives.
Vein section split open (note visible neptunite crystals imbedded in natrolite), circa 1907.
In 1912, the market for Benitoite slackened and, coming upon difficult financial times, Mr. Dallas closed the mine. The mine was re-opened briefly in 1933, and additional material was discovered.
Through the years, the mine's dumps became famous within mineral club circles as a place to collect. Occasionally, a nice sample could be found.
Between 1933 and 1967, the Dallas family leased the mine to various individuals who worked the mine for gemstones. During the 1940's, Mr. M. Hotchkiss used a bulldozer to enlarge the open cuts at the top of the hill. Between 1952 and 1967, Mr. C. Cole used a bulldozer and dynamite to continue the open cut work. Very little gem material was produced during this period.
In 1967, two new serious collectors came to the scene. Elvis “Buzz” Grey and Bill Forrest were able to secure a lease on the Benitoite Property. Grey and Forrest eventually purchased the property from the Dallas family in 1987.
From 1987 to 1999, the property was dramatically expanded, tested and mined. During this period the size of the deposit was more accurately determined and new gem resources exposed. Much excellent gemstone and specimen material was produced. In 1985, benitoite became California's State Gemstone.
Early Grizzly and Jig set-up - circa 1997
Operating the jig – circa 1997
The flurry of activity during the 80's and 90's attracted the attention of larger mining concerns including Kennecott Corporation and AZCO Mining, Inc. Both companies drilled, sampled and studied the property for its gemstone potential. The first detailed geologic picture of the mine was formulated through this work. Hundreds of feet of diamond drilling and dozens of test pits proved the existence of economical gemstone reserves.
The Benitoite Gem mine was referred to as the “Dallas mine” until the mid-1960s, but it has been variously called the “Benitoite mine,” “Dallas Benitoite mine”, and “Dallas Gem mine” (Wise and Gill, 1977). The locality is now designated the Benitoite Gem mine by the current mine owners, after the notation on topographic maps of the U.S. Geological Survey that identify the site simply as the “Gem mine.” (Laurs et al., 1997)
In 2000, the property was purchased by Benitoite Mining, Inc., a company affiliated with Collector's Edge Minerals, Inc., headquartered in Golden, Colorado. Benitoite Mining, Inc. (BMI) operated a processing plant that separated the gemstones from the properties' eluvial material. Eluvial material is the weathered remains of bedrock. The bedrock weathers naturally through time and then lies in place on the hillside. One result of the weathering was the formation of clay that would entrain the soils containing pebbles and occasional benitoite crystals.
Processing site layout
Collector’s Edge Minerals installed this new processing plant at the Benitoite Gem mine in Spring 2002. Material was dumped into the hopper (far left) and proceeded up the conveyer belt to a washing/screening apparatus (top right). Mineral specimens larger than 1.9 cm were hand picked from the conveyor belt on the far right. Smaller material was sent through a jig system. Benitoite concentrate, down to 2 mm, was removed from the jigs each day.
CEMI purchased some of their initial processing equipment from the previous owner. The existing jig equipment could handle about 25-30 tons/day delivered to it in 1 ton scoops. Work involved reprocessing the old mine dumps and mining along the weathered in-place formation not yet been mined. This material was then delivered to the processing plant and included not only the soil but organic material picked up along with the soil. Some of the boulders were size reduced while others were shipped to CEMI’s laboratory in Golden, Colorado for further processing. Old tunnels on site were opened and then dug out for processing. These tunnels went down to about 9 meters at which point there was no longer any benitoite, only natrolite.
During the period of CEMI activities (2000-2004), approximately 30,000 yards of material, including existing dump material of about 20,000 yards, were processed. One of the problems encountered was the sticky, gooey mud in which potential gem or specimen material was entrained. The existing equipment made it difficult to separate the mud from the entrained material, resulting in balls of mud passing over the screen and off the end of the conveyor belt. This required the material to be processed twice through the system to reduce losses due to the mud balls and constantly manning the conveyor belt to retrieve anything that looked promising. Another continuing problem was that the organic material would plug up the spray nozzles on the washing equipment which then required frequent cleaning. Because of the requirements of the operating permit, the small site area required processing the dump and the area under it in combination.
A new powered screen processing plant was purchased and became operational in 2002. The processing plant, which had a capacity of 150 tons/day, took advantage of the fact that benitoite is more dense than the surrounding soil which allowed it to be separated. The system also contained conditioning devices to process the mud to liberate entrained stones. The soil containing the benitoite was loaded into the hopper on the left of the photo above. The larger rocks slid off the “grizzly” (larger rocks were examined later for potential minerals), while the smaller material entered the shaker unit which then screened and washed the material, removing the lighter fractions that were discarded, leaving the heavier material (including the benitoite) behind. Material over 1.9 cm in size was sent to a conveyor belt where possible mineral specimens were removed by hand. The smaller material was sent to a jig system consisting of two large jigs that separated out material over 4 mm in size. Material equal to or smaller than 4 mm was caught in “hutch” compartments. The material from the hutches was passed on to a smaller jig that collected material down to 2 mm in size. This
“concentrate” was removed each day and sorted by hand to extract and grade the gemstone rough that was sent to cutters around the world.
A further refinement was implemented to improve gem recovery. It turns out that benitoite is slightly magnetic, due to its iron content, while none of the other material in the concentrate is. This permitted easy separation of the benitoite from the remaining concentrate, and even separation of the facet grade fractions from non-gem benitoite. Processing rates of approximately 90 kg of concentrate per hour were achieved using this method.
Bryan Lees (L), looking for gem rough and specimen material on the conveyor belt.
Picking gem benitoite from jig after screening and washing.
Close up showing benitoite (center).
Handful of gem concentrate from jigs showing pieces of gem benitoite.
Gem crystals collected from jig.
Particularly large, partially gemmy benitoite from the jig.
Besides screening the eluvium for gem material, the company also explored for benitoite mineral specimens. The locale is famous for producing world-class hand samples of this gem material. Dumps from the early 1900's were reworked to yield specimens lost during the original mining days.
View of the Benitoite Gem mine. Water was supplied from the San Benito River at the base of the workings into the pond seen below, then to the processing plant.
The excavator was used to remove overburden covering the offset vein structure. Large boulders with obvious benitoite and/or neptunite mineralization were stockpiled as the digging proceeded.
In-situ blueschist. This mineralized portion was offset from the main body.
Potential specimens were discovered during washing and screening activities. The phrase “potential specimen” is used because when discovered, the specimens are covered with a thick white coating of natrolite, a zeolite mineral that covered the original in-place benitoite veins. Fortunately, the natrolite coatings protect the benitoite and neptunite crystals on the specimens from damage during the mining and screening operations.
Boulders containing seams of natrolite containing potential gem and specimen material were cleaned prior to shipping to Collector’s Edge back in Golden, Colorado where further preparation work was done.
Boulders received at Collector’s Edge for preparation work.
Recovered mineralized boulders from old dumps to be processed at CEMI laboratory.
Once specimens were collected, each piece was carefully cleaned to remove the coating of natrolite. Occasionally, beautiful benitoite and neptunite crystals would be exposed in the process. Only a few percent of all the 'potential specimens' ever yielded a good benitoite or neptunite specimen.
Outstanding plate of natrolite containing benitoite crystals (triangular) on right, neptunite (elongated crystals on left), and twinned joaquinite crystals (brown) in the center – J. Veevaert specimen - 4 cm wide.
The Benitoite Gem mine is the only commercial source of gem-quality benitoite in the world. From the time of its discovery in 1907 until 1967, when Forrest and Gray began working the mine, it is estimated that about 2,500 carats of faceted benitoite were produced (E. Gray, pers. comm., 1997). Of that amount, nearly 1,000 carats were produced during the period of active mining between 1907 and 1911, based on the amount of rough reported in the Dallas Mining Company's account books. (Laurs et al., 1997)
Few stones exceeded 3 ct, and most of the gems weighed less than 1 ct. The balance of 1,500 carats, believed to have been recovered between 1911 and 1967, was estimated from verbal information provided by previous miners (E. Gray, pers. comm., 1997). Most of these stones were faceted by cutters in the United States, and weights of up to 5 ct were obtained. Clarence Cole retrieved most of the rough during the interval from 1952 to 1962. (Laurs et al., 1997)
From 1968 to 1997, Forrest and Gray produced approximately 2,000 carats of faceted benitoite. According to E. Gray (pers. comm., 1997), this inventory can be divided into six weight classes: (1) About 2,000 pieces were cut in commercial factories, and generally finished to less than 0.25 ct each; (2) another 1,500 pieces ranged between 0.25 and 1 ct; (3) some 500 stones weighed between 1 and 2 ct; (4) a total of 50 stones ranged between 2 and 3 ct; (5) only 25 stones weighed between 3 and 4 ct, and (6) the 15 largest stones exceeded 4 ct. Thus, 89% of the stones were under 1 ct; 9% were between 1 and 2 ct, and 2% were over 2 ct. All but the commercially cut stones were faceted by the families of the current owners or in their own facilities. Forrest and Gray did not market cuttable rough except, in a few cases, as mineral specimens. (Laurs et al., 1997)
This relatively low output, compared to other gem mining operations, demonstrates the extreme rarity of this gem material.
The New Idria district is located in the southern Diablo Range of the California Coast Range geologic province. Since 1853, the district has been mined and prospected for numerous mineral resources, including mercury, chromium, gold, asbestos, gems, and mineral specimens. The district encompasses a serpentinite body that was tectonically emplaced into surrounding sedimentary and metamorphic rocks. During the late Jurassic, when two of the earth's plates collided (see, for example, Hopson et al., 1981), the relatively low-density serpentinite rose through the overlying layers of rock, which included the Franciscan Formation. Slices of the Franciscan Formation were incorporated into the highly sheared serpentinite body during its emplacement; these are called tectonic inclusions (see Coleman, 1957). The serpentinite breached the paleo-surface in the mid- to late-Miocene (Coleman, 1961). It is now exposed along the crest of the Diablo Range over an area 23 km by 8 km, elongate to the northwest. (Laurs et al., 1997)
Tectonic inclusions of Franciscan rocks within the serpentinite consist dominantly of blueschist and graywacke, with lesser mica schist, greenstone, and amphibolite schist (Coleman, 1957; Laurs, 1995). All of these rocks are derived from oceanic crust and overlying sediments that were accreted onto the continental margin during the Jurassic period. Two blueschist protoliths (parent rocks) may be differentiated on the basis of texture and composition: (1) metavolcanic, which are largely basaltic lavas and volcaniclastics; and (2) meta-sedimentary, which are largely marine sediments. Depending on the composition of the protolith, the blueschist contains variable amounts of very ﬁne-grained albite, glaucophane-crossite, actinolite-tremolite, aegerine-augite, and titanite, with or without stilpnomelane, quartz, K-feldspar, epidote, and apatite. Benitoite is found in blueschist derived from both protoliths (Laurs, 1995; Laurs et al., 1997)
Prior to breaching the surface, the New Idria serpentinite was intruded by small bodies of igneous rocks, predominantly syenite (Coleman, 1957). The mid-Miocene age of the syenite correlates well with Neogene magmatism in the California Coast Range, which is associated with the plate tectonic reconﬁguration of western California (Van Baalen, 1995). Metamorphism associated with this reconﬁguration is probably responsible for forming calc-silicate vein assemblages that are scattered through the serpentinite, as well as benitoite-bearing veins in the blueschist bodies (Van Baalen, 1995). Tentative age data suggests that the benitoite crystallized about 12 million years ago (M. Lanphere, pers. comm., in Van Baalen, 1995), so it is much younger than the enclosing blueschist, which formed about 100 to 160 million years ago (see, for example, Lee et al., 1964). (Laurs et al., 1997)
The mine is situated on a low hill, which is underlain by Franciscan rocks emplaced into serpentinite (Coleman, 1957; Wise and Gill, 1977). These rocks consist of blueschist and greenstone, which are locally sheared. The blueschist is dark bluish gray where unaltered, and lighter blue-green within the mineralized zone. This zone is at least 60 m long, strikes N60°W, dips moderately northeast, and is about 3 m thick. Recent field observations by the authors suggest that the deposit is offset along two north-to north-west-trending faults. All of the historic lode production was obtained from the central section of the deposit. However, in the spring of 1997 an extension of the mineralized zone was discovered in the western offset portion. The faulting apparently down-dropped a portion of the mineralized blueschist, which lay buried beneath up to 10 m of unconsolidated eluvium and dump material. (Laurs et al., 1997)
This simplified geologic map (top) and cross-section (bottom) of the Benitoite Gem mine show the altered blueschist where benitoite is found, in lenses that are tectonically incorporated into the sheared greenstone. After Coleman (1957), Rohtert (1994), and Laurs (1995). This map is found in a Gems & Gemology article, Vol. 33, No. 3, p.170 written by Brendan M. Laurs, William R. Rohtert, and Michael Gray
Benitoite mineralization is confined entirely to the blueschist, in a hydrothermally altered zone, which is characterized by: (1) recrystallization of ﬁbrous amphibole and pyroxene, (2) local albite dissolution, and (3) veining and pervasive infiltration by natrolite. Benitoite formed during the first two stages of the alteration (inasmuch as many crystals contain amphibole and pyroxene inclusions), but prior to the formation of natrolite-which coats benitoite in the veins. The natrolite veins average less than 1 cm wide, and contain benitoite only locally, commonly where the veins narrow or terminate. (Laurs et al., 1997)
In-situ blueschist surrounding natrolite vein containing potential benitoite.
The benitoite forms as euhedral crystals 1-1.5 cm on average, some up to 5.6 cm in diameter, attached to the vein walls. Other minerals on the blueschist vein walls include neptunite, silica pseudomorphs after serandite, joaquinite group minerals, apatite, albite, ionesite, and the copper sulfides djurleite, digenite, and covellite (Wise and Gill, 1977), as well as traces of other minerals described by Van Baalen (1995) and Wise (1982). Natrolite coats all of the vein minerals and, in most cases, completely fills in and closes the veins. Natrolite also has inﬁltrated the altered blueschist adjacent to the veins, filling interstitial space that was previously occupied by albite. (Laurs et al., 1997)
Crystallographic drawings showing the various crystal habits and forms of benitoite. From Louderback, 1909.
Two types of benitoite were noted by Wise and Gill (1977): (1) gem-quality crystals attached to the walls of cross-cutting veins; and (2) disseminated, euhedral, non-gem benitoite with abundant amphibole and pyroxene inclusions that formed within the altered blueschist. Color zoning is common, with a milky white or colorless core grading outward into transparent blue corners. However, gem-quality blue crystals recently discovered in the western offset portion of the deposit typically lack color zoning and show only π faces. In general, the relatively small size of faceted material is due to the abundant cloudy areas and the ﬂattened morphology of the crystals. (Laurs et al., 1997)
Ever since discovery of benitoite, its origin has been a persistent enigma. The formation of benitoite at the New Idria district has been generally ascribed to hydrothermal processes that caused the unusual combination of barium (Ba) and titanium (Ti). Coleman (1957) suggested that Ti was derived from fluids associated with the crystallization of small syenite intrusions in the district; Ba was liberated from the alteration of blueschist. However, Ti is relatively immobile in hydrothermal fluids (Van Baalen, 1993), and the closest syenite to any of the benitoite deposits in the district is 1 km. Wise and Gill (1977) proposed that ﬂuids derived Ti, Fe, and rare-earth elements from the serpentine, and Ba from the blueschist, to form benitoite and the associated vein minerals. Most recently, Van Baalen (1995) proposed that benitoite formed as a result of metamorphism localized along the contact between blueschist and greenstone, in the presence of sodium-rich, low silica metamorphic ﬂuids. He suggested that more than enough Ba and Ti were present in the blueschist and greenstone to produce the inferred amount of benitoite and neptunite. (Laurs et al., 1997)
Outside the New Idria district, benitoite has been found in situ at four areas. At Big Creek-Rush Creek-in the Sierra Nevada foothills of eastern Fresno County, California-small grains of benitoite are found in gneissic metamorphic rocks near a type of igneous rock known as granodiorite (Alfors et al., 1965; Hinthorne, 1974). This occurrence is located nearly 160 km (100 miles) northeast of the Benitoite Gem mine, and is not geologically related to the New Idria district. In Japan, benitoite was noted from albite-amphibole rock in a serpentinite body, along the Kinzan-dani River, at Ohmi in the Niigata Prefecture (Komatsu et al., 1973; Chihara et al., 1974; Sakai and Akai, 1994). At Broken Hill, New South Wales, Australia, Dr. Ian R. Plimer reported benitoite as a rare mineral in high-grade granite gneiss (Worner and Mitchell, 1982). Most recently, crystals of colorless, blue, and pink benitoite averaging 1-2 mm in diameter were detected in gas cavities in syenite at the Diamond Jo quarry in Hot Springs County, Arkansas (Barwood, 1995; H. Barwood, pers. com., 1997). (Laurs et al., 1997)
Two previously reported benitoite locations should be discredited. Anten (1928) probably misidentified benitoite in a thin section from Owithe Valley, Belgium (Petrov, 1995). Lonsdale et al. (1931) tentatively identified benitoite in sediments from the Eocene Cook Mountain formation of southwest Texas; Smith (1995) suggested that the locality should be discredited because the authors confused the spelling of bentonite (a clay mineral) with benitoite, but we recommend discreditation because the available data provided by Lonsdale et al. suggest that they misidentiﬁed grains that were actually sapphirine. Rumors of benitoite from Korea have not been conﬁrmed. (Laurs et al., 1997)
Benitoite is the State Gemstone of California. Benitoite crystals vary in appearance quite widely. A small percentage of the crystals are gemmy and deep blue. The large majority of benitoite crystals are translucent with white centers and blue outer rims. While some crystals have an opaque, 'stony', blue-gray appearance due to an abundance of crossite inclusions. Most benitoite crystals are blue and strongly dichroic (colorless to deep blue). The deepest color is for light vibrating parallel to the c-axis. The color of faceted benitoite gemstones includes: colorless, pale blue, rich blue, deep purplish-blue and rarely pink.
Crystals being 2.5 cm across are considered large, and the average sizes are in the range of 1 cm to 1.5 cm. Considerable variation exists in the shapes of benitoite crystals. Benitoite most commonly forms interesting 'triply terminated' crystals, belonging to the ditrigonal-dipyramidal class.
Benitoite fluoresces blue-white under ultraviolet light. Under short wave ultraviolet light the white cores of the crystals fluoresce brighter than do the blue edges. Under long wave ultraviolet light the white cores fluoresce a dull red, while the rims of the crystals are non-fluorescent.
Benitoite crystals with 1.43 ct. gemstone.
Large specimen of benitoite and neptunite.
Benitoite and neptunite, 3.8x2.2x2 cm. John Veevaert collection.
Benitoite and neptunite, 2x1.6x1.1 cm. John Veevaert collection
Benitoite specimen, 3.8x2.7x2 cm. John Veevaert collection.
Fully terminated benitoite crystal, 1.7 cm. John Veevaert collection.
The "Wreath", one of the best known benitoite specimens, 6 cm. wide. Natural History Museum of Los Angeles County specimen.
The famous Josie Scripps benitoite, 18 cm wide. W. Larson specimen.
Superb specimen with benitoite and a 4.3 cm doubly terminated neptunite. S[eco,em 10.1 cm wide. John Veevaert specimen and photo.
Aesthetic specimen of benitoite and neptunite, 2.9x2.5x2.4 cm. John Veevaert photo and specimen.
Apatite has been found in small crystals associated with the natrolite.
Apatite crystals to 4 mm in natrolite. J. Veevaert specimen.
Djurleite is a copper sulphite closely related to chalcocite and has been found at the Benitoite Gem mine along with chalcocite.
Djurleite crystal with neptunite, 1.2 mm. C. Rewitzer specimen.
Neptunite forms lustrous, black, prismatic crystals with lengths usually 8 times their width. Interestingly, while the neptunite crystals appear to be black in most specimens, the color is actually a very deep red-brown. The crystals are most commonly attached by one end to the vein walls, but, doubly-terminated crystals attached to the vein wall by one of their prism faces are not at all rare. Crystals to 7.5 cm are known, but average crystals range up to 2.5 cm. Neptunite occasionally forms crossing twins, similar to staurolite, with the angle between the c-axis near 40 degrees.
Neptunite crystals with natrolite, 6.3 cm wide. F. Benjamin specimen.
Neptunite twinned on (301), 2.8 cm wide. W. Larson specimen.Joaquinite: Ba2NaCe2Fe2+(Ti,Nb)2Si8O26(OH,F).H2O
Joaquinite was first discovered at the Benitoite Gem mine which is considered the “Type Locality”. It occurs as tiny (up to 3 mm) yellow-brown to orange-brown equi-dimensional crystals. Pronounced “wah-keen-ite”.
Joaquinite crystal, 0.9 mm wide. C. Rewitzer specimen.
Discovered by Francis Jones in 1957 and identified in 1977 as a new species. It is considered the rarest mineral at the Benitoite Gem mine and the “Type Locality” for this species. It occurs as orthorhombic, colorless crystals, sometimes as sprays, to 3 mm in length.
Jonesite crystal cluster, 1.2 mm wide. C. Rewitzer specimen.
Commonly occurs as a white, opaque, vein-filling mineral at the Benitoite Gem mine. Crystals up to 2 cm in length with irregular shapes and curved cleavages form the solid masses. When open spaces occur, allowing for the formation of crystal faces, the form of the aggregates are certainly not typical of natrolite. Louderback (1909, p.358) described these forms as small, roof-shaped ridges commonly with curved or, more strictly, broken roof lines and coxcomb-like groups.
Natrolite crystals to 1.9 cm.
Other Vein Minerals:
Other mineral species are known to occur in the mineralized veins at the Benitoite Gem mine. The following is an alphabetized listing of those species: actinolite, albite, analcime, apatite, calcite, chrysocolla, crossite, jonesite, serandite and silica pseudomorphs after serandite. Small crystals of the following copper sulfide species are occasionally seen on specimens: chalcocite, covellite, digenite, and djurleite.
The Benitoite Gem mine in San Benito County, California is the world's only commercial source for gem benitoite.
Benitoite has a very high “Index of Refraction” (IR) and one of the highest “dispersive” powers in the mineral kingdom, making it among the most fiery, brightest gemstones in the world. These characteristics are what make benitoite such a desirable gemstone to jewelry makers the world over. Its strong blue/violet color can be enhanced by faceting the stone along certain orientations. Proper faceting is critical for this stone to show off its best features.
Benitoite crystal with faceted benitoite gemstones.
Benitoite gemstones up to 1.43 cts.
Benitoite melee faceted in full round brilliants with a consistent set of proportions. A range of colors is available, as shown here (0.07 ct each).
Provided below is a comparison of benitoite, the 'brighter blue gemstone', with Tanzanite. After studying this head-to-head comparison of several of the optical and physical properties, we believe that you will better understand why benitoite is receiving so much notoriety in major gem and jewelry publications and why benitoite has become so popular with jewelry manufacturers.
Barium Titanium Silicate
Hydrated Calcium Aluminum Silicate
One Directional Cleavage
Heat Treated to eliminate yellow-brown overtones
Trichroic (strong before heating)
Strong: 0.039, 0.046 (Diamond is 0.044)
1.69 to 1.71
Benitoite gemstones are available through Paul Cory of ITECO, Inc. at 614-923-0080 or visit ITECO's website at www.itecoinc.com
CURRENT STATUS:Benitoite Mining, Inc. (BMI), a company affiliated with Collector's Edge Minerals, Inc., headquartered in Golden, Colorado, operated the mine until 2004, selling it in 2005 to Dave Schreiner of Coalinga, California. During this time BMI ran the old mine dump material and colluvial deposits through the processing plant and mined all of the known in situ vein material. BMI then rehabilitated the site. This included covering the ponds, putting the top soil back in place and then re-seeding it, and pulling the roads back into contour. The result was a site that looked much like it did before any mining took place.
Because of current restrictions by the EPA and BLM regarding presence of naturally occurring asbestos, ATV and motorcycle damage to the area over the years, and protection of endangered plant species, access by the general public to the mine site is not allowed. The current owner is, however, allowed to mine a limited amount of material that he must transport to another location where collectors pay to go through the material in hopes of finding specimens and gems.
Early reclamation work started.
Site reclamation underway by Benitoite Mining, Inc., circa 2004.
REFERENCES:Collector's Edge Minerals, Inc. thanks Gems and Gemology, Gemological Institute of America, for permission to use portions or all of their original article from Gems & Gemology, “Benitoite from the New Idria District, San Benito County, California”, Volume 33, Number 3, pp. 166-187, Fall 1997. Contributing authors were Brendan M. Laurs, William R. Rohtert, and Michael Gray. Collector’s Edge would also like to thank John Veevaert, Trinity Minerals, for providing additional information and photographs.
Alfors J.T., Stinson M.C., Mathews R.A., Pabst A. (1965) Seven new barium minerals from eastern Fresno County, California. American Mineralogist, Vol. 50, pp. 314-340.
Anton J. (1928) Sur la composition lithologiques des psammites du Condroz. Société Géologique de Belgique, Annales, Vol. 51, pp. B330-331.
Barwood H. (1995) Benitoite and joaquinite in Arkansas. Mineral News, Vol. 11, No. 5, pp. 2, 5.
Coleman R.G. (1957) Mineralogy and petrology of the New Idria district, California. Ph.D. dissertation, Stanford University, Stanford, CA.
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