ベニト石（Benitoite)

ベニト石ルース(Faceted benitoites) 0.12 - 1.18ct

1.18ct Ø5.9mm	0.88ct 6x5.6mm	0.84ct 7.5x4.7mm	0.57ct 4.9x4.6mm	0.12ct Ø3.2mm

風化した青色片岩上のソーダ沸石中のベニト石と海王石(Benitoite & Neptunite crystal matrix in natrolite on altered blue schist)

140x110x60mm		85x54x38mm	46x32x24mm

12.5x12.5x2mm	10x9x7mm	24x13x12mm	新潟県糸魚川市青海のベニト石 　高さ(H) 3mm　宮島コレクション Benitoite crystal from Ohmi, Japan
Benitoite Gem Mine

化学組成 (Composition)	結晶系 (Crystal System)	結晶形 (Crystal Forms)			モース硬度 (Hardness)	比重 (Density)	散乱 (Dispersion)	屈折率 (Refractive Index)
BaTiSi3O9	三方晶系 (Trigonal)				6 - 6½	3.64-80	0.044	1.757-805

　Benitoite was first discovered in 1907 at 1370m elevation of Idria District, southern Diablo Range, San Benito County, California, U.S.A. Early February morning, copper and cinnnabar prospector Jim Couch noticed little flashes of light resembling reflections from broken glass, glancing across the tiny brook. Upon investigation he was astounded to see "thousands of blue gems" scattered about the hillside where they had weathered out of a snow-white material later identified as natrolite. He first thought that was sapphire. While an expert from the Los Angeles area proclaimed the new material "volcanic glass", a lapidary called the stone spinel : "It was too soft for a sapphire, so it must be a spinel, since that's the only other stone there is of that color. The lapidary showed some of the stones to George Eacret, the manager of Shre v & Co., one of the largest jewelry stores in San Francisco at that time. Eacret checked the stone and found that it was doubly refractive, so he delivered a sample to Dr. George Louderback, a mineralogist at the University of California (Berkeley) who determined that it as a new, undescribed mineral. In the company of Mr. Eacret, Dr. Louderback made his first visit to the mine on July 19, 1907, and confirmed the discovery and named the new mineral "Benitoite" upon the locality it was first discovered. He then descrived full-length article. His documentation of the ditrigonal-dipyramidal crystal habit finally proved the natural exsistence of the crystal form, which had been predicted mathematically 77 years earlier in 1830, by German mineralogist, Johann F.C.Hessel.

ベニト石の産状と特徴（Benitoite occurences and characteristics）

ベニト石の発見者ジム・カウチ Jim Couch at a campsight near Benitoite mine	ベニト石の鉱山(褐色の露頭)遠望 Benitoite Gem mine workings (small brown cut)	2000年初頭の鉱山 Mine works in early 2000
Photo ： Dallas Mining Company archive	Photocourtesy： Kennecott Exploration Company

世界のベニト石産地（Worldwide occurence of benitoite）　

　ベニト石は同じカリフォルニアのフレスノ郡、テキサス州の　Eocene Cook山、アーカンサス州の他オーストラリアBroken Hillの花崗片岩中, 日本の糸魚川流域の青海、金山谷川の蛇紋岩中の曹長石ー角閃石中等、世界の９ヶ所にて発見されています。しかしいずれも微細な鉱物標本が稀に発見されるのみです。
　実質的にはサン・ベニト郡が世界で唯一の産地と言えるでしょう。
　この産地では10kmの範囲の地域に５ヶ所の鉱床が発見されていますが、宝石級の美しい結晶の産出は質、量ともに Benitoite Gem Mine のみ。5kmほど離れた Junnila 鉱区から表面が溶融された結晶を産出し、その宝石質部分から小さな0.5ct以下のルースが僅かな量カットされることがありました。
　主力の Benitoite Gem Mine にしても産出量は極めて少量でしかありませんでした。

　Benitoite has been confirmed from nine locations around the world, but only the historic Benitoite Gem mine and the Junnila claim have produced gem-quality material. All of the commercial gem production has come from the Benitoite Gem mine. At the three other deposits in the New Idria district, benitoite has been found only as small, platy(non-gem quality) crystals, up to a few millimeters in diameter.
　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 maetamorphic rocks near a type of igneous rock known as granodiorite. This occurence is located nealy 160km northeast of the Benitoite Gem mine, and is not geologically related to the New Idria distric. In Japan, benitoite was notee from albite-amphibole rock in a serpentinite body, along the Kinzan-dani River, at Ohmi in the Niigata Prefecture. At Broken Hill, New South Wales, Australia, benitoite was reported as a rare mineal in hig-grade granite gneiss. Most recently, crystals of colorless, blue, and pink benitoite averaqging 1-2mm in diameter were detected in gas cavities in syenite at the Daimond Jo quarry inj Hott Springs County, Arkansas. Detrital grains of benitoite were recovered from Pleistocene lake sediments at the Lazzard estate, few kilometers west of Lost ills, in the San Joauin Valley, Kern County, California.
　

ベニト石ルース生産量情報（Faceted benitoite）

　1907年の発見から1967年の60年間に凡そ2500カラットのルースが生産されたと推定されます。内1000カラットが1907年から1911年の間の産出です。スミソニアン博物館所有の7.6カラットの石はこの時のものです。大半は1カラット以下の石でした。　1911年と19067年の間に1500カラットのルースが得られたと考えられます。1967年から97年までの30年間にカットされたベニト石は僅かに2000カラットにすぎません。そのうち、大きさの内訳は0.25カラット以下が2000個、0.25〜1カラットが1500個、1〜2カラットが500個、2〜3カラットが50個、3〜4カラットが25個、4カラット以上は僅かに15個でした。その他おそらく500個程度が結晶からのカット等、別ルートから流れたと推定されます。
　そんなわけで2005年に鉱山が閉山するまでの凡そ1世紀の間に生産されたベニト石は全てを足し合わせても5000カラットでしかありません。この極端な稀少さのためにベニト石が宝飾品市場に流通することは無く、限られたコレクター向けの宝石でありました。したがって値段は１カラットのルースが30年前で1000ドル、閉山となった後の現在でも1600ドルと、超稀少な宝石としては異例の妥当な水準です。

　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, it is estimated that about 2,500 carats of faceted benitoite were produced. Of that amount, nearly 1,000 carats were produced during the period of active mining between 1907 and 1911. It was during this time that 7.6 carat stone , now in the Sithsonian Museum　 was faceted. Few other stones exceeded 3 ct, however, and most of the gems weighed less than 1 ct. The balance of 1,500ct believed to have been recovered between 1911 and 1967. Since 1967 till 1997, approximately 2,000 carats of benitoite were faceted : 1) 2,000 pieces, generally less than 0.25ct, 2) 1,500 pieces between 0.25 - 1 ct, 3) some 500 stones between 1 and 2 ct 4) a total of 50 stones between 2 and 3 ct, 5) only 25 stones between 3 and 4 ct, 6) the 15 largest stones exceeded 4 ct. And 500 carats of finished goods are estimated to have entered the nmarket through informal channels, such as the cutting of older rough and the faceting from mineral specimens.
　Thus a total of about 5,000 carats of faceted benitoites have been produced over the life of the mine, which finished the mining in 2005.
　Because of this small production, benitoite has been a collector's gem, one of the rarest in the world. And the price level has been quite reasonable for the rarity, USD$1000 in early 1980's and today US$1600, even after the closure of mine in 2005.

最大2.84ctのベニト石52個(合計33ct) を使用したネックレス (A necklace with 52 benitoites)	史上最大のベニト石　15.42ct　 (The largest benitoite)	7.6ct　14.5x10.5x8mm	珍しい無色、紫、多色の例 0.34−1.29ct (Rare benitoite's color variations)
	Michael Scott Collection	Smithsonian Collection

　ベニト石の色（Benitoite coloration）　

　サファイアのような色合いのベニト石の発見以来、その発色の原因については無数の説が提唱されてきました　；　命名者ラウダーバックによるチタン還元説、コールマンによるヴァナジウム、ニオブ、銅の不純物説、レアードとアルビーの鉄の不純物説、バーンズ等の酸素原子に拘わる結晶欠陥説、同じくバーンズによるFe²⁺→Fe³⁺電荷移動説、ミレージによる金属ー金属電荷移動説やチタン側のジルコニウム説等々、ありとあらゆる説が唱えられましたがいずれも否定されています。
　ベニト石の青い色の発色の原因は未だに未解明のままです。
ベニト石の大半は青ー紫でごく稀に無色のものがあります。写真のピンクー橙の発色は加熱処理によるものです。

　Because of sapphire-like color, numerous investigators have attempted to link the chemical composition of benitoite to its coloration. Louderback(1907) initially attributed the blue color to traces of reduced Ti. Coleman(1957) suggested that trace impurities of V, Nb, and Cu might cause the blue coloration. Laird and Albee(1972) noted slightly higher concentrations of Fe in blue than in colorless benitoite. Burns et al.(1967) suggested that the coloration is caused by crystal defects due to an oxgen deficiency. Burns(1970) suggested that the pleochroism of benitoite and other minerals might be caused by Fe²⁺→Fe³⁺ charge transfer. Millage(1981) also proposed metal-metal charge transfer, such as　Fe²⁺→Ti⁴⁺　or Fe²⁺→Fe³⁺, as a coloring agent. He also attributed the blue coloration to traces of Zr in the Ti site. In spite of much effort to interprete the spectral data, the cause of color in benitoite remains elusive.

Most of natural benitoite is blue-violet and rarely colorless. Orange and pink colors are due to heat treatment.