Identification guide · Minerals
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.
Cordierite identification checklist
| Color | Blue, Violet-blue, Gray, Yellowish 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 to translucent | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 7–7.5 | 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 | Poor, one direction (pinacoidal) | |
| Fracture | Subconchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.6–2.66 | Ordinary rock-like heft (specific gravity about 2.63), similar to quartz. |
| Crystal system | Orthorhombic |
Extra tests for Cordierite
Glass check. Cordierite 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.
Cordierite look-alikes and how to tell them apart
Both sit near Mohs 7–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Cordierite vs Quartz →
Both sit near Mohs 7–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Cordierite vs Axinite →
Both sit near Mohs 7–7.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Cordierite vs Diaspore →
Cordierite is harder (Mohs 7–7.5) than Diopside (Mohs 5.5–6.5), so Cordierite will scratch Diopside and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
Cordierite identification FAQ
How can you tell if a rock is Cordierite?
Check the color (blue, violet-blue, gray, and yellowish gray), the vitreous luster, a Mohs hardness of 7–7.5 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Cordierite?
Cordierite has a Mohs hardness of 7–7.5. It scratches glass and a steel knife and cannot be scratched by a steel file.
What is the difference between Cordierite and Quartz?
Cordierite (Mohs 7–7.5, vitreous luster) and Quartz (Mohs 7, vitreous, greasy on fracture surfaces luster) look alike, but hardness, streak and heft usually separate them. See the full Cordierite vs Quartz comparison for a side-by-side table.
Is Cordierite worth anything?
Entry-level Cordierite specimens sell for around $2, typical collector pieces about $20, and premium examples $150 or more depending on size, quality and locality.
What color is Cordierite?
Cordierite is most often blue, violet-blue, gray, and yellowish gray. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
