Iolite: The Three-Color Gem and the Viking Sunstone Question

Short answer: iolite is the gem name for cordierite, a magnesium aluminum silicate (Mg2Al4Si5O18) at Mohs 7 to 7.5. Its violet-blue color comes from iron, and it is so strongly pleochroic that one stone looks blue, gray and nearly colorless from different sides. It is essentially never treated. The popular story that Vikings navigated with iolite is an untested idea: no sunstone has been found in a Viking ship or grave.

Did Vikings really navigate with iolite?

No source shows that they did. The story deserves a careful look because nearly every listing repeats it as fact.

The idea starts with a single saga. The Saga of St. Olaf describes Sigurd taking a "sunstone," looking at a snowy, overcast sky and telling the king where the hidden sun was. In the late 1960s the Danish archaeologist Ramskou proposed that such a stone could have been a birefringent crystal used as a polarizing filter: rotate it against a patch of sky until the light brightens, and the direction of the sun can be inferred from the polarization pattern.

A 2011 review in Philosophical Transactions of the Royal Society B by Gábor Horváth and colleagues (PMC) sets out where things stand. The authors write that it is not clear what the sunstone was made of, and that it could have been cordierite or tourmaline, both common in Scandinavia. They also note that the theory had been accepted and frequently cited in spite of a total lack of experimental evidence, which is why they went out to measure skylight polarization over clear, cloudy and foggy skies. Their results show the physics can work under some skies, especially with the sun low, which makes the method plausible. Plausible is not the same as proven.

The only physical candidate ever recovered is not iolite. In 2013 a French and British team argued that the Alderney Crystal, a block of Icelandic calcite found in a 16th-century wreck in the English Channel, could have served as a solar compass. The Associated Press report of that work adds two caveats: no such crystals have ever been recovered from Viking tombs or ships, and Donna Heddle of the University of the Highlands and Islands called the hypothesis speculative. The ship was Elizabethan, centuries after the Viking Age, and the stone was calcite, not cordierite.

GIA handles the tale with the right hedge: "According to legend," iolite is the Viking Compass Stone, and the gem is attractive "whether or not it ever served as a navigation tool." That is the honest version. Iolite could have worked as a polarizer, Iceland spar could have, tourmaline could have, and the saga does not say which.

One naming trap is worth knowing: the feldspar gem now sold as sunstone has nothing to do with the saga's sunstone. Same word, different stone.

What is iolite, mineralogically?

Iolite is the transparent gem form of cordierite. The mineral name honors the French geologist Pierre Louis Antoine Cordier (1777 to 1861); "iolite" comes from the Greek ios, violet.

Values are from GIA's iolite entry, the International Gem Society and webmineral, which lists the old synonym dichroite and gives the species a pre-IMA date of 1813 with Bodenmais, Germany as the locality. Minerals.net adds that cordierite forms a solid solution series with the rare iron-rich mineral sekaninaite, with cordierite as the magnesium end, and that pure iron-free cordierite is uncommon.

Why does iolite change color as you turn it?

Pleochroism means a crystal absorbs light differently along different directions. Cordierite has three optical directions and shows a different color in each. Minerals.net calls iolite the most pleochroic gemstone, with the effect clearly visible in every transparent stone.

GIA's description page breaks down what you will actually see. Violet iolites show light violet, dark violet and yellow-brown. Bluish iolites show colorless to yellow, blue-gray and dark violet. A bluish stone can look completely colorless or yellow from one side, and a violet one can look brown. That is also the origin of the misleading trade name "water sapphire," which GIA notes was applied incorrectly to iolite.

For a cutter this is the whole game. IGS describes iolite as frequently step cut and often cut shallow to lighten its tone, with the rough oriented to put the blue face-up. Turn a finished stone on its side and you will see the gray or straw color the cutter worked to hide.

What gives iolite its color?

Iron. GIA states that the violet-to-blue hue comes from iron in the structure, and that the deep blue of some iolites is thought to share sapphire's cause, an iron and titanium charge transfer. The wider mechanisms behind mineral color are in the planned color causes guide.

Where does iolite come from, and how does it form?

Cordierite is a mineral of heated aluminum-rich rocks. Minerals.net lists contact metamorphic rocks such as hornfels and gneiss, altered serpentine environments and coarse granites, with common companions of quartz, biotite, almandine, andalusite, corundum and talc. It is a classic product of clay-rich sediments cooked at high temperature and modest pressure, the setting explained in the planned guide to metamorphic minerals.

Because it is hard and resists weathering, iolite also concentrates in river gravels. GIA names the gem gravels of Sri Lanka, Kenya and central Tanzania, India, Brazil and Norway, and notes a significant deposit discovered in Madagascar in 1994. IGS says most iolite in commerce comes from India, with substantial amounts from Tanzania, Brazil and Sri Lanka.

Specimen localities are a different list. Minerals.net cites large altered crystals from the Silberberg Mine at Bodenmais in Bavaria, well-formed crystals from Søndeled near Risør in Norway, gem cordierite in the Tranomaro area of Madagascar, Babati in Tanzania and Virgolândia in Minas Gerais, and big opaque crystals from a talc quarry in Richmond, New Hampshire. Japan supplies an oddity: flower-shaped muscovite pseudomorphs after cordierite from Kameoka, known as "cherry blossom stones."

How do you tell iolite from sapphire, tanzanite and kyanite?

The rotation test settles most cases. Hold the stone or tumble under a single white light and turn it slowly. Iolite shifts from violet-blue to gray or straw yellow, often dramatically. Minerals.net's comparison list gives the rest:

• Sapphire: harder (9) and lacks iolite's strong color shift.
• Tanzanite: also pleochroic, but with higher refractive index and specific gravity; it is the zoisite variety known only from the Arusha area of Tanzania. GIA notes gemologists first suspected tanzanite might be cordierite when it appeared.
• Kyanite: rarely transparent, lower in hardness, and without the purplish tinge.
• Amethyst: a deeper purple and no blue-to-yellow swing.

A ten-power loupe helps too; see using a loupe. GIA notes that properly oriented inclusions can give iolite a cat's-eye or aventurescence, a glittery sparkle, so a spangled stone is not automatically a fake.

What drives iolite's price?

Color first, then size. GIA rates violetish blue to fine blue as the most desirable range and says fine faceted iolites over 5 carats are rare. IGS quotes Sinkankas's wholesale figures of $60 to $80 per carat for fine 1 to 5 carat stones and $100 to $150 for 5 to 10 carats, and notes stones over 8 carats are rarely eye clean. Ordinary material is described as steely, inky or washed out. Because iolite cannot be heated without damage (GIA cites its low melting point), what you see is natural color, which some buyers value.

Is iolite linked to any traditions?

In crystal traditions iolite is associated with intuition and inner vision, likely an echo of the navigation legend. GIA lists it as the gem for the 21st wedding anniversary. Treat both as custom, not mineral properties.

Choosing iolite in our stock

Small tumbles are the most affordable way to see pleochroism for yourself, and a rough piece of a second pleochroic mineral makes a useful comparison.

1. Rotate before you judge color. A good tumble shows a strong violet-blue face and a gray or yellow side; a stone that stays one color in every direction is worth questioning.
2. Look for translucency. Light should pass through thin edges; iolite tumbles that look opaque and inky are the lower grade IGS describes.
3. Expect natural color. No treatment is known, so "enhanced iolite" is a label to ask about.
4. Ignore Viking claims in the description. A seller who presents the legend as history is not a seller to trust on locality either.

• Iolite: an inexpensive tumbled iolite, the easiest way to test the color shift under a single lamp.
• Blue Kyanite: bladed blue kyanite, the stone most often confused with iolite; compare the streaky blades with iolite's glassy body.
• Blue Kyanite with Black Mica Rough Wand: another metamorphic blue, with mica intergrowth that shows the kind of rock these minerals grow in.
• Blue Kyanite spheres and palm stones: polished 55 to 67 mm spheres and palm stones, a contrast in luster and fibrous structure to iolite's glassiness.

Frequently asked questions

Is iolite the same as cordierite?

Yes. Iolite is the trade name for transparent gem cordierite, Mg2Al4Si5O18, usually with some iron. Older books also call it dichroite. Opaque or altered cordierite in rock is still cordierite, but it is not cut as iolite.

Is iolite a Viking sunstone?

It might have been one candidate, but nothing proves it. A 2011 Royal Society review said the sunstone's material is unknown and could have been cordierite or tourmaline. The only recovered candidate, the Alderney Crystal, is calcite from a 16th-century wreck, and no sunstone has been found in a Viking ship or grave.

Why is iolite called water sapphire?

Because a good violet-blue stone can resemble sapphire. GIA describes the name as incorrect: iolite is a different mineral, much softer than sapphire's 9, and it turns gray or yellow when rotated, which sapphire does not do so strongly.

Is iolite treated or dyed?

No routine treatment is known. GIA says iolite cannot be heated to deepen its blue because its low melting point will not tolerate it, and IGS notes no treatments have succeeded in lightening color or removing inclusions. Natural color is the norm.

Is iolite good for everyday jewelry?

Reasonably. At 7 to 7.5 it resists scratches, but GIA notes distinct cleavage in one direction gives it only fair toughness, so a ring stone can break with a hard knock. Earrings and pendants are the safer settings.

Sources

• GIA, Iolite: https://www.gia.edu/iolite
• GIA, Iolite History and Lore: https://www.gia.edu/iolite-history-lore
• GIA, Iolite Description: https://www.gia.edu/iolite-description
• International Gem Society, Iolite Value, Price, and Jewelry Information: https://www.gemsociety.org/article/iolite-jewelry-and-gemstone-information/
• Minerals.net, Cordierite: https://www.minerals.net/mineral/cordierite.aspx
• Minerals.net, Iolite: https://www.minerals.net/gemstone/iolite_gemstone.aspx
• Webmineral, Cordierite Mineral Data: http://webmineral.com/data/Cordierite.shtml
• Horváth et al., On the trail of Vikings with polarized skylight, Philosophical Transactions of the Royal Society B (2011), PMC: https://pmc.ncbi.nlm.nih.gov/articles/PMC3049005/
• R&D World (Associated Press), Stone in shipwreck may be Viking navigators' tool: https://www.rdworldonline.com/stone-in-shipwreck-may-be-viking-navigators-tool/