Identification guide · Minerals
How to Identify Corundum
Corundum forms hexagonal, barrel-shaped or tabular crystals with a vitreous to slightly adamantine luster and a colorless to white streak regardless of body color. Its hardness of 9 is the single most diagnostic property, since it will scratch every common mineral except diamond and a handful of rare synthetics. Color ranges from colorless through red, blue, yellow, and purple, and some crystals show a six-rayed star effect called asterism when cut as a cabochon, caused by tiny oriented needle-like inclusions of rutile.
Corundum identification checklist
| Color | Red (ruby), Blue (sapphire), Colorless, Yellow, Green, Pink, Gray | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Adamantine to 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 | 9 | It scratches glass, steel and even quartz; only harder gems will mark it. |
| Streak | White (colorless) | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | None (rhombohedral/basal parting) | |
| Fracture | Conchoidal to uneven | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 3.95–4.10 | Heavier than it looks (specific gravity about 4.025). |
| Crystal system | Trigonal |
Extra tests for Corundum
Glass check. Corundum 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.
Corundum look-alikes and how to tell them apart
Corundum is harder (Mohs 9) than Calcite (Mohs 3), so Corundum will scratch Calcite and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Corundum vs Calcite →
Corundum is harder (Mohs 9) than Beryl (Mohs 7.5–8), so Corundum will scratch Beryl and not the other way round. It scratches glass, steel and even quartz; only harder gems will mark it. Corundum vs Beryl →
Both sit near Mohs 9, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Corundum vs Chrysoberyl →
Corundum is harder (Mohs 9) than Feldspar (Mohs 6–6.5), so Corundum will scratch Feldspar 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.
Corundum identification FAQ
How can you tell if a rock is Corundum?
Check the color (red (ruby), blue (sapphire), colorless, and yellow), the adamantine to vitreous luster, a Mohs hardness of 9 and a white (colorless) streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Corundum?
Corundum has a Mohs hardness of 9. It scratches glass, steel and even quartz; only harder gems will mark it.
What is the difference between Corundum and Calcite?
Corundum (Mohs 9, adamantine to vitreous luster) and Calcite (Mohs 3, vitreous luster) look alike, but hardness, streak and heft usually separate them. See the full Corundum vs Calcite comparison for a side-by-side table.
Is Corundum worth anything?
Entry-level Corundum 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 Corundum?
Corundum is most often red (ruby), blue (sapphire), colorless, and yellow. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
