* cordierite gemstone
Iolite: identification, value, and how to spot a fake
Gem cordierite. Strong trichroism — looks blue, light blue, and yellow from three angles. Called the 'Viking compass stone' for purported use as a polarizing filter in cloudy navigation.
Published May 30, 2026
Quick facts
- Mineral family
- cordierite
- Chemical formula
- (Mg,Fe)2Al4Si5O18
- Mohs hardness
- 7–7.5
- Refractive index
- 1.522–1.578
- Specific gravity
- 2.58–2.66
- Crystal system
- orthorhombic
- Luster
- vitreous
Colors
Iolite is found in blue-violet, with strong trichroism. The body color, its saturation, and how evenly it spreads through the stone are among the strongest drivers of value.
How to tell real from imitation
No single test settles it, but a few grounded checks quickly narrow things down for Iolite. Hardness, brilliance, and the list of stones it is commonly confused with all give you something concrete to look for.
- Scratch resistance: genuine Iolite sits at Mohs 7–7.5, so it should resist scratching from softer materials — a stone that mars far too easily is suspect.
- Brilliance & luster: Iolite shows a vitreous luster and a refractive index of 1.522–1.578; imitations often look glassier or flatter under the same light.
- Density: a real Iolite has a specific gravity near 2.58–2.66, so it feels heavier or lighter in the hand than many look-alikes of the same size.
- Commonly imitated by: tanzanite, blue sapphire. If a deal looks too good, suspect one of these substitutes first.
- Fast first read: photograph the stone in the Jewelry Identifier app to estimate the gemstone and its likely value range, then confirm anything important with a gemological lab.
Common treatments
- none typically
Treatments are routine for many Iolite stones, but they materially affect value and should always be disclosed — an untreated, lab-certified stone commands a clear premium over a treated one.
What to know about Iolite
Gem cordierite. Strong trichroism — looks blue, light blue, and yellow from three angles. Called the 'Viking compass stone' for purported use as a polarizing filter in cloudy navigation.
Sources
* Frequently asked
FAQ
- Q. Is Iolite hard enough for everyday rings?
- A. Mostly. At a Mohs hardness of 7–7.5, Iolite stands up to routine wear, though sharp knocks and abrasive dust can still mar it over years of daily use.
- Q. How can I tell Iolite from tanzanite?
- A. Iolite and tanzanite can look alike to the naked eye, but they differ in measurable properties — refractive index (1.522–1.578 for Iolite), specific gravity (2.58–2.66), and hardness (Mohs 7–7.5). A gemologist confirms the distinction with a refractometer and loupe; the Jewelry Identifier app gives a fast first read from a photo.
- Q. Does Iolite come in colors other than blue-violet?
- A. Yes. Iolite occurs in blue-violet, with strong trichroism. Color is driven by trace elements and treatment history, and it is one of the biggest factors in a stone's value.
* Try it
Identify the jewelry in your hand, right now.
Just take a photo — AI reads the metal, gemstone, hallmark, era, and an estimated value range in seconds. First two scans free, no account required.
Download on the App Store* Related