Tourmaline: Properties, Varieties, Origins and Value

Tourmaline is not one mineral but a family of them, boron silicates built on the same framework with room in it for almost any element the ground has to offer. The framework does not care what it takes in, and each guest is a colour: manganese pink, iron green and blue, chromium green, copper the neon blue that made the family famous. A single crystal can change recipe as it grows and come out pink at the core and green at the rim. No other gem species gives so many colours, and none is priced across so wide a range, from ornamental to a multiple of fine sapphire.

What it is

The group is defined by a general formula, XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W, in which each letter is a site that different elements can fill. Nearly all gem tourmaline is elbaite, the lithium-bearing species that gives pink, red, green, blue and the multicoloured crystals. Dravite carries magnesium and gives the browns and, with chromium and vanadium, the chrome greens of East Africa. Liddicoatite, from Madagascar, gives the polychrome slices with triangular zoning. Schorl is the black iron tourmaline that fills museum drawers and is rarely cut. Crystals are long prisms with a rounded triangular cross-section and deep striations down their length, in the trigonal system. Hardness 7 to 7.5, no cleavage, specific gravity around 3.06 for elbaite. Refractive index 1.624 to 1.644 with a strong birefringence near 0.020, and a pleochroism strong enough to govern the cutting: most tourmaline is darkest looking down the length of the crystal, so a cutter orients the table along the prism to show the lighter, brighter colour, and a stone cut the wrong way goes nearly black.

Where the colours come from

Manganese gives pink and, at concentration, the red of rubellite. Iron gives green and, in a different state, the blue of indicolite, and too much of it gives the dark greens that heat is used to lighten. Chromium and vanadium together give the chrome green of Tanzania and Kenya, a colour that rivals tsavorite and emerald in daylight. Copper with manganese gives the Paraíba blue and green, an electric saturation nothing else in the family reaches, and the variety has its own note. Titanium and colour centres fill in the rest. Because the crystal takes on whatever the fluid carries as it grows, zoning is normal: watermelon tourmaline, pink inside and green outside, is a growth history you can wear.

Where it comes from

Brazil is the reference. Minas Gerais has produced fine rubellite, indicolite, green and bicolour tourmaline for two centuries from pegmatites around Governador Valadares and Araçuaí, and the state of Paraíba gave the family its most valuable stone. Mozambique, from the pegmatites of Nampula province, produces large clean crystals in most colours and the African Paraíba-type material. Nigeria supplies pink, red and copper-bearing stones. Afghanistan, from Nuristan, gives fine pink and green; Pakistan, Namibia and Madagascar contribute. Tanzania and Kenya are the sources of chrome tourmaline. Maine and California produced the historic American material and still yield a little. Origin rarely moves the price except for Paraíba, where Brazilian origin carries a premium that a laboratory can support with trace chemistry.

How it is judged

Colour, on the three axes, with the family’s particular failures in mind. Rubellite must be red without brown; a great deal of it goes brownish in incandescent light, and the stones that hold their red under a lamp are the ones worth paying for. Indicolite must be blue without the grey that iron brings, and it is rare in medium tone; most is dark. Green tourmaline of ordinary iron colour is abundant and cheap; chrome green is not. Then clarity, where red and pink stones are often included and greens and blues expected clean. Then the cut, because pleochroism and the long crystal shape push cutters toward deep, narrow stones that go dark, and the arithmetic of recutting applies. Then size, and tourmaline reaches sizes corundum never does, so weight rewards less except in Paraíba, where anything above three carats of top colour is an event.

Treatments, and what to demand on paper

Heat is common and gentle: it lightens over-dark greens and blues and removes the violet component from copper-bearing stones to leave pure blue, at temperatures low enough to leave little evidence, so laboratories often write that heating cannot be excluded. Irradiation deepens pink to red and is stable. Both are accepted when disclosed. Fracture filling with resin appears on some Paraíba and must be stated. Coatings and dyes surface on cheap goods. For most tourmaline a report is a formality; for Paraíba it is the purchase, because copper has to be measured and origin has to be argued, and the report should state both separately.

The market

Two markets that share a name. Ordinary tourmaline is priced as fine jewellery material by colour and size, with rubellite, indicolite and chrome at the top and green and bicolour goods below. Paraíba is priced like a great coloured stone: the finest small Brazilian stones trade per carat at levels beside fine unheated sapphire, and larger Mozambican stones of top colour sit not far behind. There is no colour appellation for the family except the word Paraíba itself, which the laboratories have defined as copper-bearing blue to green elbaite regardless of origin, with the origin stated on its own line. That definition is where the money is argued.

Care

Hardness 7 to 7.5 and no cleavage make tourmaline a reliable stone for rings with ordinary care. Included stones should stay out of the ultrasonic; heated and filled stones out of the torch. The family is pyroelectric: a stone warms in the sun or under a lamp and attracts dust, which is why old dealers keep a cloth beside the tray.

Where I stand

Tourmaline is not the core of this office, and most of the family is honest jewellery material that needs no hunter. Two members are different. Chrome tourmaline from East Africa is a green I will put beside tsavorite for a client who wants the colour at a different price. And Paraíba is a stone I hunt on mandate with the same discipline as ruby, because it is priced like ruby, faked like ruby and argued over like ruby, and the note that follows this one sets out why.

Questions collectors ask

What is the most valuable tourmaline?

Copper-bearing Paraíba, in the neon blue to green the trade calls electric, and above all the tiny Brazilian stones from the original deposit. After it, fine rubellite of true red without brown, clean saturated indicolite, and chrome tourmaline from East Africa. In every variety saturation without grey is what the price follows.

Is tourmaline treated?

Often, and mildly. Heat lightens over-dark greens and blues and removes violet from Paraíba, and it leaves little trace; irradiation deepens pink and red. Both are accepted when disclosed. Fracture filling appears on some Paraíba and must be on the report. Nothing like the heating industry behind corundum exists for tourmaline.

Why does tourmaline come in so many colours?

Because its structure has room for almost any element. The same crystal framework takes manganese for pink, iron for green and blue, chromium and vanadium for chrome green, copper for Paraíba, and often changes recipe as it grows, which is why one crystal can be pink at the core and green at the rim.