Benitoite Gem Mine
Benitoite Gem Mine, San Benito County, California: found in 1907, the Dallas mine until the 1960s. Six species sheets name it; its crystals sit on natrolite.
The Benitoite Gem Mine, in the New Idria district of San Benito County, California, is where benitoite was found in early 1907. Called the Dallas mine until the mid-1960s, the Benitoite Gem Mine had yielded about 5,000 carats of faceted stones by 1997, and Gems & Gemology then called the Benitoite Gem Mine the only commercial source of gem benitoite.
In short
- Found in 1907, named for the county. Gems & Gemology (Laurs, Rohtert and Gray, 1997) says benitoite was discovered in early 1907 and named after San Benito County, the San Benito River and San Benito Mountain. The IMA list gives benitoite 1907, USA. Who found it is disputed: Louderback credited L. B. Hawkins and T. E. Sanders in 1907 and named J. M. Couch in 1909.
- Six Handbook sheets name the Gem mine. By script over the Handbook of Mineralogy sheets read for this page: benitoite, neptunite, joaquinite-(Ce), natrolite, serandite and andradite. Cinnabar, jadeite and glaucophane, among others, name New Idria, Clear Creek or Panoche Valley in San Benito County, but not the Gem mine.
- The production ledger, worked. Laurs et al. give about 2,500 ct of faceted benitoite from 1907 to 1967, about 2,000 ct from Forrest and Gray after 1967 and about 500 ct through informal channels: 5,000 ct in all. Of the 4,090 stones they class by weight, 590 (14.4%) exceed 1 ct and 90 (2.2%) exceed 2 ct.
- Conflicting accounts, printed side by side. The naming year (1907 or 1909), the end of mining (1912 or 2005) and the operator (Forrest and Gray, or the “Benitoite Mining Company”) differ between sources. The table and sections below attribute each one.
- What this page does not say. Nothing about whether anyone may visit or collect there today. The latest sources fetched are Gem-A, July 2020, and an undated state museum page; both are quoted with their dates, and neither is a current statement.
| Species | Formula (IMA unless marked) | Hardness | Density | 20.00 g piece in water | Handbook wording for the district | IMA status, year, country |
|---|---|---|---|---|---|---|
| Benitoite | BaTiSi3O9 | 6–6.5 | 3.65 | 14.52 g | “at the Gem mine, Mina Numero Uno, the Victor and Junilla claims, and on Santa Rita Peak” | G, 1907, USA |
| Neptunite | KNa2LiFe2+2Ti2Si8O24 | 5–6 | 3.19–3.23 | 13.73–13.81 g | “splendid crystals from the Gem mine and Mina Numero Uno” | G, 1893 |
| Joaquinite-(Ce) | NaBa2Ce2Fe2+Ti2Si8O26(OH)·H2O (sheet) | 5.5 | 3.89–3.98 | 14.86–14.97 g | “At the Gem mine and to its north, on Santa Rita peak; at Mina Numero Uno and on the Victor claim” | Rd, 2001 s.p., USA |
| Natrolite | Na2(Si3Al2)O10·2H2O | 5–5.5 | 2.20–2.26 | 10.91–11.15 g | “at the Gem mine, San Benito Co.” | A, 1997 s.p., Germany |
| Serandite | NaMn2+2Si3O8(OH) | 5–5.5 | 3.34 | 14.01 g | “from the Gem mine, San Benito Co.” | Rn, 1931, Guinea |
| Andradite | Ca3Fe3+2(SiO4)3 | 6.5–7 | 3.8–3.9 | 14.74–14.87 g | “around the Gem mine, San Benito Co.” | G, 1868, Norway |
Where the Benitoite Gem Mine is, and what kind of deposit it is
Position. Laurs et al. (Gems & Gemology 33, 1997) put the Benitoite Gem mine 32 km (20 miles) northwest of Coalinga, California, at about 1,380 m, in the New Idria district of the southern Diablo Range. The district is a serpentinite body exposed over about 23 km by 8 km, mined and prospected since 1853 for mercury, chromium, gold, asbestos, gems and specimens.
The deposit. In the district, Laurs et al. say, benitoite occurs only in blueschist bodies caught up in the serpentinite. At the mine the mineralised zone is at least 60 m long and about 3 m thick. The crystals grew on the walls of natrolite veins, averaging under 1 cm wide, commonly where the veins narrow or end, and natrolite later coated them. Tentative age data put the benitoite at about 12 million years, much younger than the 100 to 160 million-year-old blueschist around it.
Its neighbours. Laurs et al. place the Junnila claim 7 km, Mina Numero Uno 7.8 km and the Victor claim 9.2 km northwest of the mine, and Santa Rita Peak 1.2 km north-northeast; the benitoite sheet names all four. Other American localities on the site, Franklin and Sterling Hill among them, are on the localities index.
Who found benitoite, and when? The disputed discovery
Early 1907, with two stories. Laurs et al. say the literature agrees on early 1907 but not on the finder. Louderback (1907) credited L. B. Hawkins and T. E. Sanders; two years later he named J. M. Couch, a prospector grubstaked by R. W. Dallas, and quoted both sides as claiming the discovery. Couch’s son published The Benitoite Story in 1961 for his father’s sole claim. Gem-A’s 2020 article names Couch as discoverer.
What Couch thought he had. Laurs et al. say a sapphire deposit; Gem-A says blue diamonds. The California State Capitol Museum page says benitoite is also known as the “blue diamond”. We print all three and choose none.
When it was named. Laurs et al. record Louderback’s first visit to the mine on 19 July 1907; the sheet’s first reference is his 1907 paper; Gem-A gives 30 July 1907 for the naming report. The Capitol Museum says benitoite was given its name two years after the 1907 discovery, which conflicts with the other three. On labels, what a type locality means is set out separately; the sheet records type material as n.d.
Who has owned and worked the Benitoite Gem Mine? 1907 to 1997
The Dallas years. Laurs et al.: the Dallas Mining Company was formed soon after the discovery and the property called the Dallas mine; active mining began in July 1907; most benitoite was broken off under its natrolite coat for gem rough, so few well-crystallised specimens were recovered in that period. Operations ceased in 1912, the equipment was auctioned in October 1913, and the mine received patent papers in 1917. The Dallas family owned it until 1987.
Leases. Little activity from 1920 to 1940; Miller Hotchkiss leased it in the 1940s and was first to bring a bulldozer; Clarence Cole held a lease from 1952 to 1967 and sublet in 1966 to Gerold Bosley. After Cole died in 1967 the lease passed to William Forrest and Elvis (Buzz) Gray, who bought the mine from the Dallas family in 1987 and were the sole owners in 1997. A further extension of the deposit was found down slope of the old pit in spring 1997.
Later accounts conflict. Gem-A (July 2020), citing Wilson (2008), says the mine operated in some capacity from 1907 to 2005 under several names and owners, and that the site was reclaimed and sold in 2005. The Capitol Museum page, undated, says it has been mined by the Benitoite Mining Company since the discovery. Neither date nor owner can be reconciled from what we fetched, so both are printed.
How much benitoite has the mine produced? The production ledger
The 1997 figures. Laurs et al. estimate about 2,500 ct of faceted benitoite from discovery to 1967: nearly 1,000 ct from 1907 to 1911, from the Dallas Mining Company’s account books, and about 1,500 ct from 1911 to 1967 on miners’ verbal reports. Forrest and Gray had produced about 2,000 ct since 1967, and about 500 ct more reached the market through other channels. Total: about 5,000 ct over the mine’s life to 1997.
Worked from their weight classes. Forrest and Gray’s stones: about 2,000 under 0.25 ct, 1,500 from 0.25 to 1 ct, 500 from 1 to 2 ct, 50 from 2 to 3 ct, 25 from 3 to 4 ct and 15 over 4 ct. That is 4,090 stones, of which 590 (14.4%) exceed 1 ct and 90 (2.2%) exceed 2 ct. The first five years produced about a fifth of the total carats; the thirty years after 1967, about two-fifths.
Read the ledger as estimates. Half the pre-1967 figure rests on verbal information, and the authors call every figure approximate. The largest cut stone in their table is 15.42 ct. The species itself is on benitoite.
Which minerals the Handbook places at the Gem mine
Six sheets. Method: a script reads the Distribution field of each cached Handbook of Mineralogy sheet (pdftotext -layout, whitespace joined) for Gem mine and San Benito. Six name the Gem mine, as tabled above. That is the sheets read for this page, not a species list for the deposit; the species pages are on the species index: Laurs et al. also name jonesite, apatite, albite and the copper sulphides djurleite, digenite and covellite on the vein walls.
One disagreement. The serandite sheet names the Gem mine; Laurs et al. describe the material there as silica pseudomorphs after serandite. A Gem-mine serandite label may therefore be a pseudomorph; see what is a pseudomorph.
Documented negatives. The cinnabar sheet names New Idria, the jadeite sheet Clear Creek near New Idria, and the glaucophane sheet Glaucophane Ridge, Panoche Valley: all San Benito County, none the Gem mine. A label reading only San Benito Co. does not mean the Gem mine. The white vein mineral, natrolite, reads 10.91 to 11.15 g in water for a 20.00 g piece, against 14.52 g for benitoite and 13.73 to 13.81 g for neptunite.
Can you visit the Benitoite Gem Mine? What the dated sources say
In 1997. Laurs et al. describe the mine as 16.2 hectares (40 acres) of private patented mining property behind a locked gate, patrolled by law enforcement, with written permission from the owners required to enter. They say the four other benitoite prospects in the district were closed to the public. The owners then worked only in the spring months.
In 2020. Gem-A wrote in July 2020 that the site was then open as a small pay-to-dig operation. That is a statement about 2020, and we found no later source.
Today. We make no claim about present ownership, access, collecting or digging; check with the owner of record before relying on any of the above. On labels, Benitoite Gem mine, New Idria district, San Benito Co., California is what the sheets and Laurs et al. support; see how to label and catalogue a mineral collection. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.