Cordierite
Cordierite is a magnesium iron aluminum cyclosilicate, (Mg,Fe)2Al4Si5O18, best known for its strong pleochroism, appearing blue-violet, gray, or pale yellow depending on the angle from which it is viewed. Its transparent gem variety, called iolite, has long been nicknamed 'water sapphire' for its violet-blue color. It crystallizes in the orthorhombic system and typically forms stubby, pseudo-hexagonal crystals.
Cordierite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | (Mg,Fe)2Al4Si5O18 |
| Mohs hardness | 7–7.5 |
| Crystal system | Orthorhombic |
| Luster | Vitreous |
| Streak | White |
| Cleavage | Poor, one direction (pinacoidal) |
| Fracture | Subconchoidal |
| Specific gravity | 2.6–2.66 |
| Transparency | Transparent to translucent |
| Common colors | Blue, Violet-blue, Gray, Yellowish gray |
| Rarity | common |
Geology & background
Cordierite develops mainly through contact and regional metamorphism of aluminum-rich sedimentary rocks, particularly shales, forming in hornfels, schist, and gneiss alongside andalusite, sillimanite, garnet, and biotite. It also crystallizes directly from magma in some granites and pegmatites. Notable gem-quality iolite comes from Sri Lanka, Madagascar, and India, while metamorphic cordierite in less transparent form is widespread across regionally metamorphosed terrains such as New England and parts of Scandinavia. Collectors seeking gem iolite should test pleochroism by rotating a specimen; a shift from deep blue-violet to pale yellow or gray along different axes is essentially diagnostic. Weathered cordierite readily alters to a soft, greenish mica-like mineral called pinite, so fresh, glassy material is less common than its wide geologic distribution might suggest.
How to identify Cordierite
Cordierite forms grayish-blue to violet, glassy crystals with a vitreous luster and conchoidal fracture, and cleavage is poor. Hardness is notably high for a silicate at 7 to 7.5, similar to quartz, which helps separate it from softer look-alikes. Its standout trait is strong pleochroism, meaning a single crystal can look deep blue-violet from one direction and pale yellow-gray from another. It is often confused with quartz or sapphire until this color shift, or its lower density than sapphire, is checked.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Cordierite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Cordierite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Cordierite's gem form, iolite, was nicknamed 'water sapphire' by early gem traders because of its rich blue-violet color.
- Viking sailors are traditionally said to have used cordierite crystals as a polarizing 'sunstone' to help locate the sun on overcast days.
- A single cordierite crystal can appear blue-violet, gray, or pale yellow depending on the direction from which it is viewed.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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