Bi-colour Tourmaline

Silicates — tourmaline group · also called Parti-coloured tourmaline, Watermelon tourmaline, Bicolor tourmaline

Bi-colour tourmaline is a single elbaite crystal carrying two or more distinct colours as growth zones, most often green and pink. The zoning may run along the prism or from core to rim, the latter producing the sliced watermelon stones. Hardness is 7 to 7½ and the material is untreated more often than tourmaline generally.

What is bi-colour tourmaline?

Bi-colour tourmaline is a single tourmaline crystal, in almost every case of the species elbaite, that shows two or more distinct colours within one continuous piece of material. The colours are growth zones rather than separate stones, and the boundary between them may be knife-sharp or graded over several millimetres.

The zoning takes two forms. Longitudinal zoning runs along the length of the prism, so a crystal can be green at one end and pink at the other, and cutters lay these out as long step cuts that carry both colours in the table. Concentric zoning runs from core to rim, so a slice across the prism shows a pink centre inside a green skin — the arrangement the trade calls watermelon tourmaline. Parti-coloured is the older term for the same material. The group these stones belong to is covered in the general tourmaline entry.

What makes one tourmaline crystal grow two colours?

Tourmaline records the chemistry of the fluid it grew in. As a pegmatite pocket cooled and the residual melt changed composition, the crystal went on growing and each new layer took up whatever colouring elements were then available. A shift in iron or manganese content is preserved in the finished crystal as a shift in colour.

Pink and red in elbaite are attributed to manganese, principally Mn3+ generated where manganese-bearing material has been exposed to natural radiation over geological time. Green is due chiefly to iron, blue to iron in mixed valence states, and the yellows and browns to combinations of these with titanium. Because iron tends to be depleted early in a pegmatite's evolution while lithium and manganese concentrate in the late fluid, a very large number of crystals are iron-green at the base and manganese-pink at the tip. The pattern is the ordinary case, not an accident.

How is bi-colour tourmaline identified?

Elbaite reads a refractive index of roughly 1.619 to 1.655 with a birefringence near 0.018 to 0.020, is uniaxial negative, and has a specific gravity around 3.01 to 3.11. Those figures are constant across the colour zones of a single stone, which is itself diagnostic: a genuine bi-colour gives one reading, not two.

Pleochroism is strong, the extraordinary ray markedly darker than the ordinary, and it is the reason so many tourmalines are cut with the table parallel to the length of the crystal. Under magnification the material typically shows fine hollow tubes and thread-like liquid-filled channels running parallel to the c-axis, along with irregular liquid-and-gas feathers. What the stone must be separated from is an assembly. A doublet or triplet of two coloured glasses, or of two pieces of quartz cemented together, imitates the effect cheaply; immersion, a look at the girdle under magnification for a cement plane, and a refractometer reading taken on more than one facet will expose it.

The distinguishing feature: a polar axis that grows two different ends

Tourmaline is hemimorphic — the two ends of the crystal are not crystallographically equivalent. One termination is structurally different from the other, the two grow at different rates and accept different elements, and the result is a mineral naturally disposed to colour zoning in a way that stones of higher symmetry are not. Bi-colour tourmaline is that asymmetry made visible.

The same polar structure produces two physical curiosities. Tourmaline is pyroelectric, developing an electrical charge along the c-axis when heated or cooled, and piezoelectric under pressure — which is why a warm tourmaline in a shop window collects dust and paper fragments. No other common gem behaves in this way, and the effect was recorded in Europe long before anyone understood the crystallography behind it. It also explains why a colour boundary in tourmaline is normally a plane perpendicular to the length of the crystal, whereas colour zoning in sapphire follows a quite different geometry.

Is bi-colour tourmaline treated?

Tourmaline as a group is routinely heated to lighten over-dark green and blue material, and irradiated to deepen pale pink and red. Both treatments are stable and neither is detectable with certainty by ordinary gemmological testing. Applied to a bi-colour stone, however, they act on the zones unequally, and the usual reason for leaving a fine parti-coloured crystal alone is that treatment blurs the balance the stone is bought for.

Clarity enhancement is the more pressing question. Included tourmaline is regularly filled with resin or oil to subdue surface-reaching fractures, and this must be disclosed. Sliced watermelon material is sometimes backed or stabilised to hold a fragile wafer together. Dyeing into fractures is uncommon but not unknown. Tourmaline is not produced synthetically on any commercial scale, so a convincing bi-colour is a natural stone or an assembled imitation, not a laboratory-grown crystal.

Where does bi-colour tourmaline come from?

Brazil is the historic and still the principal source, above all the pegmatite districts of Minas Gerais around Araçuaí and Governador Valadares. Afghanistan and Pakistan supply the Nuristan and Kunar pegmatites; Madagascar, Mozambique, Nigeria and Namibia are all steady producers; and Russia's Malkhan field in Transbaikalia yields fine pink-and-green material.

The United States has a place in this that is out of proportion to its output. The Himalaya Mine at Mesa Grande and the pegmatites of the Pala district in San Diego County, California, produced the pink-cored, green-rimmed crystals that made watermelon tourmaline known in the first place, and Mount Mica and the Newry pegmatites in Maine have yielded parti-coloured crystals since the nineteenth century. Madagascar deserves a separate note: many of the spectacular triangular zoned slices sold as tourmaline from Anjanabonoina are liddicoatite, the calcium-rich species named after Richard T. Liddicoat, rather than elbaite.

Where the name comes from

Tourmaline descends from the Sinhalese turamali, a term Sri Lankan dealers used for parcels of assorted gem gravels whose contents had not been separated — an origin that suits a mineral group of unusual chemical variety. Elbaite is named for the island of Elba, where the species was described from the pegmatites at San Piero in Campo.

Elba supplies the oldest named bi-colour habit as well. Its pale crystals capped with a black termination have been called testa di moro, or moor's head, since long before the chemistry was understood, and green-capped crystals from the same locality were called testa di turco. Watermelon is a much later and purely descriptive trade term, and parti-coloured, borrowed from heraldry, was the standard English usage through the nineteenth century.

What sizes are available?

Bi-colour tourmaline is one of the few genuinely rare effects available in large sizes. Pegmatite pockets produce prisms of considerable length, and clean faceted stones of five to twenty carats are obtainable, with larger ones regularly cut. Polished watermelon slices reach several centimetres across.

Size alone therefore does very little for the price, and the curve is shallow by the standards of coloured stones. What is scarce is not a big bi-colour but a well-placed one: a stone in which both colours are bright, the division falls where the cutter intended, and the crystal has survived being sawn across a zone boundary. Small calibrated bi-colours are difficult for the opposite reason — a colour break has to be positioned within a few millimetres — so the useful sizes tend to begin around three carats. Cutting rough is discussed further under rough.

How much is bi-colour tourmaline worth?

Placement of the colour break comes first: a division that sits centrally, or in a deliberate proportion, and runs cleanly across the stone is worth a considerable multiple of one that wanders into a corner. The crispness of the boundary is next, then the individual quality of each colour, since a fine pink against a muddy olive is worth less than two moderate colours in agreement.

Clarity follows, and here the market is tolerant: tourmaline is expected to carry some included material, though surface-reaching fractures at a zone boundary are a genuine defect rather than a cosmetic one. Cut quality matters unusually much, because the cutter is solving for pleochroism, colour placement and yield at the same time. Size is the weakest factor of the five. Watermelon slices are priced as objects rather than as gems, on the symmetry and centring of the pattern. None of this touches the copper-bearing material, whose pricing is separate and covered under Paraíba tourmaline.

Is bi-colour tourmaline durable enough to wear?

At 7 to 7½ on the Mohs scale, with no true cleavage, tourmaline is a practical stone for everyday jewellery. It is nonetheless brittle, and the internal strain associated with a sharp change in composition means a zone boundary is the plane along which a stone will part if it is knocked or thermally shocked.

Sudden temperature change is the specific risk. Ultrasonic cleaners and steam should not be used, both because of that strain and because included tourmaline is so often resin-filled; warm water, mild detergent and a soft brush are enough. A protective setting is worth having for a ring, particularly on a long step-cut stone whose corners are exposed. The pyroelectric effect means the stone attracts dust more readily than most, so it will look dull sooner between cleanings than a sapphire in the same setting.

How to buy bi-colour tourmaline without being caught out

Settle four things: that the stone is one crystal and not an assembly, that the colour placement is as good as the price assumes, that any clarity enhancement has been disclosed, and that the boundary is free of surface-reaching fractures. The first is answered by immersion and a careful look at the girdle for a cement plane.

Examine the stone face-up in diffuse daylight rather than under a spotlight, which exaggerates contrast and can make a weak division look decisive. Tilt it, because a break that reads as central through the table often proves to sit well off-centre in the pavilion. Ask directly whether the stone has been oiled or resin-filled, and treat vagueness as an answer. For sliced watermelon material, check whether the wafer has been backed or stabilised and whether it is thick enough to set. Where value warrants it, ask for a report from a recognised laboratory such as GIA, SSEF or Gem-A, or arrange independent testing through our services.

Looking to buy bi-colour tourmaline? See the collection, the rough, or send us a brief.