Iolite is the gem name for transparent cordierite, a magnesium-iron-aluminium silicate of the formula (Mg,Fe)2Al4Si5O18. It crystallises in the orthorhombic system, has a hardness of 7 to 7.5 on the Mohs scale, a refractive index of approximately 1.53 to 1.55, a birefringence of around 0.008 to 0.012 and a specific gravity near 2.6. The colour ranges from a soft greyish blue to a deep violet-blue, and the species is famous in gemmology for its strong pleochroism, which gives the trade name dichroite still occasionally encountered in older literature.

The pleochroic personality

The defining optical character of iolite is pleochroism so strong that it is visible to the naked eye. Looking down one axis the stone shows a deep violet-blue; looking down a second axis it shows a pale yellowish or colourless body; looking down the third axis it shows a slightly different blue. This trichroism is the gem's signature, and it dictates how the stone must be cut. A cutter who orients the table in the wrong direction produces a pale, washed-out gem; the same rough oriented along the strong colour axis yields a vivid violet-blue stone. The mineral name dichroite, in older texts, captured the visible character but is technically a misnomer because three colours, not two, are present.

The Viking sun-stone tradition

A long-standing tradition in popular gemmological writing connects iolite with Viking navigation. The story holds that polarised light passing through a calibrated piece of iolite would allow a navigator to locate the position of the sun even through cloud, by virtue of the mineral's selective transmission of polarised light along specific axes. The historical evidence for this practice is debated; archaeological finds of cordierite in Viking-era contexts are limited, and the textual evidence in the sagas is allusive rather than specific. The story is widely repeated in trade material but should be treated as plausible folklore rather than established history.

Sources and supply

Iolite is produced from a number of metamorphic and pegmatitic deposits worldwide. India has been an important source, with material from Tamil Nadu and Andhra Pradesh circulating in the trade for decades. Sri Lanka produces gem-quality cordierite from its alluvial deposits, often as part of the broader Ratnapura suite. Madagascar, Tanzania, Brazil, Myanmar, Namibia and Wyoming in the United States all yield iolite. The species is comparatively abundant, and finished prices remain moderate compared with sapphire and tanzanite, the other blue gems with which it competes.

Inclusions and identification

Iolite typically contains inclusions of zircon, sometimes producing pleochroic haloes around the zircon grains because of radiation damage. Hematite and goethite inclusions occasionally produce a reddish iridescent variety known as bloodshot iolite, which is rare and prized by collectors. Identification follows refractive index, specific gravity, and the unmistakable strong trichroism. The strong pleochroism distinguishes iolite from sapphire, blue spinel, tanzanite and synthetic blue spinel, none of which shows trichroism of the same intensity.

Cutting and durability

The cutter must orient the stone to capture the strongest blue along the table direction. Round and oval cuts are common, with emerald cuts and cushion cuts also widely seen. Iolite has a distinct cleavage in one direction that requires care during setting; bezel and protective settings are sometimes preferred for ring use, although the species is durable enough for everyday wear with reasonable care. Heat sensitivity is moderate; rapid temperature changes can cause damage along the cleavage plane.

Trade position

Iolite is positioned in the middle of the coloured-stone market, more affordable than sapphire and tanzanite, more expensive and more interesting than commercial blue topaz. The species suits buyers who want a strong violet-blue colour with genuine character at a moderate price, and its growing presence in designer jewellery has helped raise its profile over the past two decades. The combination of sufficient hardness, strong colour, ample supply and reasonable pricing has kept iolite a consistent presence in the trade since the late twentieth century.