Achroite

Tourmaline · also called Colourless tourmaline, White tourmaline, Achroite tourmaline

Achroite is the colourless variety of elbaite, the lithium tourmaline species behind most gem tourmaline. It is defined by an absence: iron, manganese, chromium and copper all too low to absorb visible light. Its high birefringence produces facet doubling visible through a loupe, which separates it from every other colourless stone of similar refractive index.

What is achroite?

Achroite is the colourless variety of elbaite, the lithium-bearing species that supplies almost all gem tourmaline. It is defined by absence rather than presence: a tourmaline in which iron, manganese, titanium, chromium, vanadium and copper have all fallen low enough that no visible absorption is left behind.

The tourmaline structure is unusually hospitable to transition metals, which is why the group produces a wider range of colours than any other gem family. Achroite is what remains when a pegmatite pocket has effectively run out of colouring elements — the last, chemically cleanest fraction of a crystallising melt. Most stones offered under the name are in fact very faintly pink, green or brown; material that stays water-clear under both daylight and incandescent light is the exception rather than the rule. The wider species is set out on the tourmaline hub and in our tourmaline entry.

Why is achroite colourless?

Colour in elbaite is produced by transition-metal ions and by charge transfer between them: iron for blues and dark greens, manganese for pinks and reds, a manganese–titanium interaction for many yellows and browns, copper for the neon blues. Remove all of them and nothing absorbs in the visible range, so the stone transmits the whole spectrum.

That is a demanding chemical condition. Pegmatite fluids almost always carry some iron, and iron alone is enough to introduce a pale green or grey cast at a few hundred parts per million. Achroite therefore forms where the melt has been thoroughly fractionated and the earlier crystals have already scavenged the metals. On Elba, where the variety was first described, colourless zones commonly appear as caps on the termination of otherwise coloured crystals — the last material to grow, from the most depleted remaining fluid. The same sequence explains why colourless zones so often sit alongside pale pink ones in a single crystal.

Note that colourless is not the same as inert. Achroite still has the full tourmaline absorption behaviour in the ultraviolet and infrared; it simply has nothing acting between roughly 400 and 700 nanometres.

How is achroite identified?

The refractive index and birefringence settle it quickly. Elbaite reads roughly 1.619 to 1.655 with a birefringence around 0.018, and low-iron colourless material sits near the bottom of that range. No other common colourless gemstone combines a refractive index near 1.62 with a birefringence that high.

Danburite is the closest competitor on refractive index at 1.630 to 1.636, but its birefringence is only about 0.006 and its specific gravity near 3.00. Colourless topaz reads 1.609 to 1.643 with a birefringence of 0.008 to 0.010 and a specific gravity of 3.53, far heavier. Goshenite, the colourless beryl, reads 1.577 to 1.583 at 2.72. Rock crystal quartz reads 1.544 to 1.553 at 2.65. Phenakite is the only one that comes close on birefringence at about 0.016, and its specific gravity of roughly 2.95 separates it from tourmaline's 3.03 to 3.10.

Optic character is the third confirmation: tourmaline is uniaxial negative, which excludes the biaxial stones — topaz, danburite — outright.

The distinguishing feature: doubling you can see

A birefringence of 0.018 is high enough to be visible through a loupe. Look through the table of a faceted achroite at the opposite pavilion facet junctions and the edges appear doubled, cleanly and unmistakably, in a way that no colourless quartz, beryl, topaz or danburite of the same size will show.

This single observation does most of the work in the field. It also flags the imitation that matters commercially: colourless cubic zirconia and glass are singly refractive and show no doubling at all, and colourless zircon doubles far more strongly still, at a birefringence around 0.059, so its facet edges look blurred rather than merely paired.

Tourmaline's second habit trait is worth knowing even though it is easier to see in rough. The species is both pyroelectric and piezoelectric: warm a crystal or apply pressure to it and it develops a charge at the terminations that visibly attracts dust and paper fragments. Museum cases holding tourmaline collect dust at the crystal tips for this reason.

Is achroite treated?

Achroite itself is generally not treated, for the obvious reason that there is nothing to improve. The relevant treatment question runs the other way: pale and colourless elbaite is the standard feedstock for irradiation.

Gamma irradiation of low-manganese-state elbaite produces pink and red by creating colour centres, and the result is stable in ordinary light and effectively undetectable by any routine test. A significant share of inexpensive pink tourmaline on the market began as near-colourless rough. Heating works in the opposite direction and will lighten or remove a manganese-related pink, which is one reason a deep natural pink should never be exposed to a jeweller's torch without protection.

For a buyer of achroite the practical consequence is small but real. Colourless material has an alternative use at a higher price, so genuinely fine, clean, well-cut colourless tourmaline is scarcer in the market than the abundance of the raw material would suggest.

Where does achroite come from?

Elba, in the Tuscan archipelago, is the type locality and gave elbaite its name. The San Piero and Sant'Ilario pegmatites produced the crystals with pale and colourless terminations that were described in the nineteenth century; the deposits are historically important and now largely worked out and protected.

Madagascar produces colourless and near-colourless elbaite from the pegmatite belts of the central highlands, and Brazil does the same from Minas Gerais, where the great lithium pegmatites yield the full colour range and the colourless fraction as a by-product. In the United States, the pegmatites of San Diego County in California — the Himalaya, Tourmaline Queen and neighbouring workings — have produced colourless zones alongside their better-known pink and bicoloured material. Mozambique and Nigeria contribute smaller quantities from more recent African pegmatite finds.

None of these is a source for achroite in the way a mine can be a source for sapphire. It is everywhere a minor product of pockets being worked for colour.

Where the name comes from

The name is Greek: *a-* for without and *chroa* for colour, giving achroos, colourless. It entered mineralogical use in the early nineteenth century alongside the other colour-based tourmaline varietal names — rubellite for the pink and red, indicolite for the blue, verdelite for the green, schorl for the common black iron species.

These names are descriptive trade and varietal terms rather than mineral species. The species is elbaite, named in 1913 after Elba, and a laboratory report will normally say so, listing achroite as a variety name if it uses it at all. Some laboratories decline to use the varietal names entirely and report only the species with a colour description, which is a defensible position and one worth expecting rather than disputing.

What sizes are available?

Larger than the price would suggest. Elbaite grows in long prismatic crystals and colourless zones are often the cleanest part of them, so faceted stones of five, ten and twenty carats exist and are not difficult to find. Size is not the limiting factor here; demand is.

That inverts the usual price logic. In most species the per-carat price climbs steeply with weight because large clean rough is scarce. In achroite the per-carat price stays close to flat, and above about ten carats it can fall, because a large colourless tourmaline is a harder thing to sell than a small one and a cutter is unlikely to recover the extra labour.

How much is achroite worth?

Little, in absolute terms, and that is a straightforward reflection of the market. The colourless gem market is dominated by diamond at the top and by inexpensive synthetics at the bottom, and a stone with a refractive index of 1.62 and a dispersion of 0.017 returns neither the brilliance nor the fire that buyers of colourless stones are looking for.

Value therefore rests almost entirely on cutting and clarity. A precisely cut, eye-clean achroite with good symmetry is worth a multiple of an average one, because the cutting is doing all of the work the material cannot do by itself. Locality adds nothing measurable except for Elba specimens, and those trade as mineral specimens for their provenance rather than as gems. Anyone paying a serious per-carat price for a colourless tourmaline is paying for craft, and should be able to see it.

Is achroite durable enough to wear?

Yes, comfortably. At 7 to 7.5 on the Mohs scale tourmaline is harder than quartz, so it resists the abrasion caused by ordinary airborne dust that dulls softer stones. Cleavage is absent to indistinct, so the stone will not part along a plane under a knock.

The two real cautions are thermal and mechanical. Tourmaline dislikes sudden temperature change, and long included stones with liquid-filled capillaries can develop fractures if heated quickly — a jeweller's torch near an unprotected stone is the usual way this happens. Ultrasonic and steam cleaning should be avoided on any tourmaline with visible inclusions.

Warm water, mild soap and a soft brush are sufficient. Because colourless stones show grease and soap film far more obviously than coloured ones, achroite needs cleaning more often to look its best.

How to buy achroite without being caught out

Check the doubling first. A loupe through the table, looking for paired facet junctions, takes seconds and separates achroite from the colourless quartz, glass and cubic zirconia it is most often confused with — none of which double, and zircon, which doubles far too much.

Then look at the stone under two light sources rather than one. Very pale pink and pale green tourmaline can pass for colourless under warm shop lighting and reveal its tint outdoors, and the two are not priced the same.

Ask what the stone is, in species terms, and expect the answer elbaite. If a report is supplied, read it for the species line rather than the variety name, since laboratories differ in whether they use the term achroite at all. Finally, judge the cut hard, because in a colourless stone with modest optical properties there is nothing else to judge. If you want us to look for a particular size or cut, tell us what you are after, or see our gems.

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