Identification guide · Semi-Precious Stones
How to Identify Iolite
Iolite is usually violet-blue, blue, and gray with a vitreous luster and is transparent. It has a Mohs hardness of 7. It scratches glass and a steel knife and cannot be scratched by a steel file. Its streak is white. It is most often confused with Cordierite, Garnet, and Tanzanite; the tests below separate them.
Iolite identification checklist
| Color | Violet-Blue, Blue, Gray | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Transparent | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 7 | It scratches glass and a steel knife and cannot be scratched by a steel file. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Good | |
| Fracture | Conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.6 | Ordinary rock-like heft (specific gravity about 2.6), similar to quartz. |
| Crystal system | Orthorhombic |
Extra tests for Iolite
Glass check. Iolite shares a glassy luster and conchoidal fracture with man-made glass. Glass sits near Mohs 5.5 and often contains round gas bubbles, so a hardness test and a hand lens separate them.
Iolite look-alikes and how to tell them apart
Both sit near Mohs 7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Iolite vs Cordierite →
Both sit near Mohs 7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Iolite vs Garnet →
Both sit near Mohs 7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Iolite vs Tanzanite →
Spinel is harder (Mohs 8) than Iolite (Mohs 7), so Spinel will scratch Iolite and not the other way round. It scratches glass and a steel knife and cannot be scratched by a steel file.
Is my Iolite real?
- Hardness: real Iolite sits at Mohs 7. Glass imitations are about 5.5, so genuine Iolite scratches glass while a glass fake cannot.
- Bubbles: round gas bubbles under a 10x loupe mean glass or resin, never a natural stone.
- Color: dyed material concentrates color in cracks and pores; look for pigment collecting at the edges of fractures.
- Temperature: natural crystals feel cool and take longer to warm in your hand than plastic or resin.
Field tips
- Check whether the specimen's color and texture match typical Iolite examples.
- Use hardness, streak, luster, grain size, and locality together; no single trait is enough for every specimen.
Iolite identification FAQ
How can you tell if a rock is Iolite?
Check the color (violet-blue, blue, and gray), the vitreous luster, a Mohs hardness of 7 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Iolite?
Iolite has a Mohs hardness of 7. It scratches glass and a steel knife and cannot be scratched by a steel file.
What is the difference between Iolite and Cordierite?
Iolite (Mohs 7, vitreous luster) and Cordierite (Mohs 7–7.5, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Iolite vs Cordierite comparison for a side-by-side table.
How do I know if my Iolite is real?
Real Iolite matches the hardness listed above, shows no round gas bubbles under magnification, and does not concentrate dye in cracks. Glass and resin fakes fail at least one of those checks.
Is Iolite worth anything?
Entry-level Iolite specimens sell for around $30, typical collector pieces about $100, and premium examples $800 or more depending on size, quality and locality.
What color is Iolite?
Iolite is most often violet-blue, blue, and gray. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
