Comparison · Minerals vs Semi-Precious Stones
Cordierite vs Iolite
Cordierite and Iolite are easy to confuse. Both sit near Mohs 7–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Cordierite and Iolite
- Both sit near Mohs 7–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
- Hold a thin edge to the light: Cordierite is Transparent to translucent, Iolite is transparent.
- They overlap in blue, purple, and gray, but Cordierite also comes in yellow.
- Well-formed crystals give it away: Cordierite is Orthorhombic, Iolite is orthorhombic.
Cordierite vs Iolite properties
| Property | Cordierite | Iolite |
|---|---|---|
| Type | Minerals | Semi-Precious Stones |
| Mohs hardness | 7–7.5 | 7 |
| Luster | Vitreous | vitreous |
| Streak | White | White |
| Transparency | Transparent to translucent | transparent |
| Specific gravity | 2.6–2.66 | 2.6 |
| Crystal system | Orthorhombic | orthorhombic |
| Common colors | Blue, Violet-blue, Gray, Yellowish gray | Violet-Blue, Blue, Gray |
| Rarity | common | uncommon |
| Typical specimen value | $2–$150+ | $30–$800+ |
Which is more valuable?
Iolite is usually the more valuable of the two: a typical collector specimen runs about $100 against $20 for Cordierite, and premium Iolite can reach $800 or more.
About 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.
About Iolite
Iolite is the gem variety of cordierite, showing strong pleochroism—appearing different colors from different angles. Vikings may have used it for navigation.
Cordierite vs Iolite FAQ
Is Cordierite the same as Iolite?
No. Cordierite is a mineral while Iolite is a semi-precious stone. Both sit near Mohs 7–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
How can you tell Cordierite from Iolite?
Both sit near Mohs 7–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Hold a thin edge to the light: Cordierite is Transparent to translucent, Iolite is transparent. They overlap in blue, purple, and gray, but Cordierite also comes in yellow.
Which is harder, Cordierite or Iolite?
They are about equally hard, both near Mohs 7–7.5.
Which is more valuable, Cordierite or Iolite?
Iolite is usually the more valuable of the two: a typical collector specimen runs about $100 against $20 for Cordierite, and premium Iolite can reach $800 or more.
