Benitoite

Silicates · also called Blue diamond of California, California state gem

Benitoite is a rare barium titanium silicate, found as transparent blue crystals in commercial quantity at a single mine in San Benito County, California. Hexagonal, hard 6 to 6½, it combines sapphire-blue colour with dispersion higher than diamond's and an intense blue-white fluorescence under shortwave ultraviolet.

What is benitoite?

Benitoite is a barium titanium silicate, BaTi(Si3O9), which occurs as transparent blue crystals in gem quality at a single deposit in San Benito County, California. It crystallises in the hexagonal system, in a symmetry class it occupied alone for decades, and its dispersion is higher than that of diamond.

It is one of the genuinely rare gemstones rather than one of the merely expensive ones. The mineral was found in 1907 and has since been recorded at a handful of other localities, but everywhere except the Californian deposit it appears as microscopic crystals of interest to mineralogists and to nobody else. Every faceted benitoite of any consequence has come from a few acres of serpentinite in the New Idria district, and since commercial working there ceased in 2005 the trade has been supplied largely by material cut before that date.

What gives benitoite its blue colour?

The blue arises from charge transfer between iron and titanium, the same mechanism that colours blue sapphire. Titanium is an essential constituent of benitoite and iron enters in traces, so the intensity of colour tracks the iron content. Material runs from colourless through pale grey-blue to a saturated indigo.

What complicates matters is that benitoite is strongly dichroic. The ordinary ray is very nearly colourless and the extraordinary ray deep blue, so the same crystal looks almost white down one direction and richly coloured across another. The habit works against the cutter: crystals are flattened triangular tablets, and the strong colour lies along the direction in which the crystal has least material to give. Orienting a stone for colour therefore costs weight, and orienting it for weight costs colour. A pale benitoite is often a well-formed crystal cut the easy way.

How is benitoite identified?

The refractive indices, about 1.757 to 1.804, sit near the top of a standard refractometer's range, and the birefringence of roughly 0.047 is high enough that the doubling of back facets is obvious through a loupe. Benitoite is uniaxial positive, with a specific gravity between about 3.64 and 3.68.

Those figures separate it cleanly from what it is mistaken for. Blue sapphire is uniaxial negative with a birefringence near 0.008 and shows no doubling. Tanzanite is biaxial and strongly trichroic, with much lower indices. Blue zircon reads far higher and doubles more strongly still. Iolite and blue spinel are lower again. Dispersion settles any remaining doubt: at about 0.046 benitoite throws visible fire, which no blue sapphire does, and the combination of that fire with a sapphire-like blue is not a thing any other stone offers.

The distinguishing feature: fluorescence and a crystal class of its own

Under shortwave ultraviolet, benitoite fluoresces an intense chalky blue-white that is unmistakable and that none of its look-alikes reproduces. It is also a crystallographic singularity: it was the first mineral described in the ditrigonal-dipyramidal class, and its flattened triangular crystals display that unusual symmetry directly to the eye.

The fluorescence is the field test, and it is the reason collectors keep a shortwave lamp on the table at mineral shows. Under longwave the reaction is weak or absent, which is itself informative. The crystallography matters less commercially but explains the habit: the tabular triangular plates, alternately larger and smaller in section, are the outward expression of a symmetry that for a long time had no other mineral in it. The classic matrix specimen, blue benitoite with black blades of neptunite in white natrolite, comes from this deposit and no other, and joaquinite, a further mineral of the same association, was first described from it.

Is benitoite treated?

Benitoite reaches the market untreated as a matter of course. No routine enhancement is established for it, and none would be commercially sensible for a stone whose value rests on being what it is. The things to guard against are a synthetic that exists in small quantity and blue simulants sold under a loosely spelled version of the name.

Synthetic benitoite has been grown, chiefly in Japan, in crystals small enough that the material has never been a market force; a cut stone of any size is overwhelmingly likely to be natural, but a laboratory report remains the answer where value justifies one. Specimen preparation is a separate disclosure question: many matrix pieces have had their natrolite dissolved with acid to expose the benitoite and neptunite beneath, which is normal practice and is not a fault, but a buyer is entitled to be told.

Where does benitoite come from?

In gem quality, from one place: the Benitoite Gem Mine, originally the Dallas Gem Mine, at the headwaters of the San Benito River in San Benito County, California. The crystals occur in veins in blueschist blocks enclosed in serpentinite of the Franciscan Complex, in the New Idria district.

Other occurrences exist and none of them produces cuttable stones. Benitoite has been recorded at Ohmi in Japan and at Magnet Cove in Arkansas, in both cases as minute crystals. The Californian mine was worked intermittently from its discovery and closed to commercial mining in 2005; since then the ground has been open at intervals to fee-digging, which yields small quantities of specimen and melee-sized material rather than the sort of rough that makes a two-carat stone. The practical consequence is that the supply is a fixed above-ground stock which is not being replaced.

Where the name comes from

The mineral is named for San Benito County, and through the county for the San Benito River at whose headwaters it was found. It was described in 1907 by George D. Louderback of the University of California, working on crystals brought in by the prospector James M. Couch, who had taken them for sapphire.

The county name is the Spanish for Saint Benedict. Couch's misidentification was reasonable enough — a hard, transparent, sapphire-blue crystal from a Californian hillside had no other obvious explanation — and it was Louderback who established that the material was a new species and went on to describe joaquinite from the same association. Benitoite was adopted as the state gem of California in 1985, which is a fair reflection of the fact that the state holds the only deposit of it that has ever mattered.

What sizes are available?

Benitoite is a small stone. The great majority of faceted material weighs well under half a carat, a clean stone above one carat is unusual, above two carats it is rare, and the largest reported cut stones are in the region of fifteen carats. The tabular habit limits yield before the crystal size does.

The price curve reflects that geometry rather than any scarcity of the mineral in the abstract. There is a clear step at one carat and a much sharper one at two, because the orientation problem means a two-carat stone of full colour has consumed a crystal far more than twice the size of the one behind a one-carat stone. Melee is available and comparatively affordable. What is genuinely difficult to assemble is a matched suite, since colour, tone and cut orientation all have to agree across several stones drawn from a supply that is no longer being added to.

How much is benitoite worth?

Colour comes first: an even, saturated blue without a grey component carries a large premium over the pale material that makes up most of the production. Size follows, with the steps described above. Clarity and cutting quality come next, and cutting matters more here than for most stones.

It matters more because three properties pull against each other. Dichroism decides whether the stone shows its colour face-up; high birefringence means a careless orientation puts visible doubling through the pavilion; and the dispersion that gives benitoite its fire is only released by a well-proportioned cut. A badly oriented benitoite can be pale, sleepy and doubled all at once, and is worth a fraction of a good one of the same weight. Crystals compete with cut stones for the same rough: a fine benitoite on natrolite matrix is often worth more left alone than cut, which is why the best pieces stay on specimens shelves rather than reaching the gems trade.

Is benitoite durable enough to wear?

At 6 to 6½ on the Mohs scale benitoite is softer than quartz, which means ordinary dust will abrade it over time. It is also brittle and has an indistinct cleavage. That combination makes it a reasonable stone for pendants, earrings and brooches and a demanding one for a ring worn daily.

Where a ring is wanted, a protective setting that shields the girdle and a bezel or half-bezel rather than exposed claws is the sensible answer, and the wearer should expect to have the stone repolished eventually. Never clean benitoite in an ultrasonic tank or with steam: brittleness and included material make both a real risk. Warm water, a mild detergent and a soft brush are sufficient, and the stone should be kept away from other gems in storage so that harder material does not scratch it.

How to buy benitoite without being caught out

Establish three things: that the stone is benitoite and not a blue substitute, that the colour is genuine face-up colour rather than an artefact of the lighting, and that the price reflects where the stone sits on a curve that steps hard at one and two carats. A shortwave lamp answers the first question in seconds.

Look at the stone under a loupe for the doubling of the back facets, which confirms the high birefringence and rules out sapphire and spinel immediately. Judge colour in diffuse daylight rather than under a jeweller's spotlight, because dispersion flatters a pale stone badly under a point source. Ask what the cut has cost the colour, and turn the stone off-axis to see how quickly the blue drains away. Treat any offer of large, cheap, deeply coloured benitoite as a claim requiring evidence, since the supply that would have produced it stopped nearly twenty years ago. For a stone of consequence, ask for a report from a recognised laboratory such as GIA or Gem-A, or arrange independent testing through our services; for comparison of fire against another dispersive rarity, see demantoid.

Looking to buy benitoite? See the collection, the rough, or send us a brief.