Comparison · Minerals
Corundum vs Diaspore
Corundum and Diaspore are easy to confuse because both are minerals. Corundum is harder (Mohs 9) than Diaspore (Mohs 6.5–7), so Corundum will scratch Diaspore and not the other way round. It scratches glass and a steel knife and cannot be scratched by a steel file. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Corundum and Diaspore
- Corundum is harder (Mohs 9) than Diaspore (Mohs 6.5–7), so Corundum will scratch Diaspore and not the other way round. It scratches glass and a steel knife and cannot be scratched by a steel file.
- Streak color differs: Corundum leaves a white (colorless) streak on unglazed porcelain, Diaspore leaves white.
- Corundum feels noticeably heavier for its size (specific gravity 3.95–4.10 versus 3.3–3.5 for Diaspore).
- They overlap in clear, yellow, green, pink, and gray, but Corundum also comes in red and blue and Diaspore also comes in brown and purple.
- Well-formed crystals give it away: Corundum is Trigonal, Diaspore is Orthorhombic.
Corundum vs Diaspore properties
| Property | Corundum | Diaspore |
|---|---|---|
| Type | Minerals | Minerals |
| Mohs hardness | 9 | 6.5–7 |
| Luster | Adamantine to vitreous | Vitreous to pearly on cleavage |
| Streak | White (colorless) | White |
| Transparency | Transparent to translucent | Transparent to translucent |
| Specific gravity | 3.95–4.10 | 3.3–3.5 |
| Crystal system | Trigonal | Orthorhombic |
| Common colors | Red (ruby), Blue (sapphire), Colorless, Yellow, Green, Pink, Gray | Colorless, White, Gray, Yellowish, Brown, Green, Violet-pink (color-change) |
| Rarity | common | common |
| Typical specimen value | $2–$150+ | $2–$150+ |
Which is more valuable?
Typical specimens of Corundum and Diaspore trade in a similar range, around $20.
About Corundum
Corundum is aluminum oxide, Al2O3, crystallizing in the trigonal system and ranking as the second-hardest natural mineral after diamond. Its gem varieties, red ruby and every other color of sapphire, are colored by trace amounts of chromium, iron, or titanium substituting into an otherwise colorless crystal structure. Non-gem corundum is widely used as an industrial abrasive under the name emery.
About Diaspore
Diaspore is aluminum oxide hydroxide, AlO(OH), a hard, dense mineral that forms in aluminum-rich metamorphic and hydrothermal environments and ranks among the principal minerals in bauxite ore. It is dimorphous with boehmite, sharing an identical chemical formula but a different crystal structure, and it characteristically splits apart with a sudden crackling sound when heated, a property that inspired its name. Transparent gem-quality crystals, sold as zultanite or csarite, display a striking color change from green to pink or champagne under different lighting.
Corundum vs Diaspore FAQ
Is Corundum the same as Diaspore?
No. Both are minerals, but they are distinct species with different hardness, streak or luster. Corundum is harder (Mohs 9) than Diaspore (Mohs 6.5–7), so Corundum will scratch Diaspore and not the other way round. It scratches glass and a steel knife and cannot be scratched by a steel file.
How can you tell Corundum from Diaspore?
Corundum is harder (Mohs 9) than Diaspore (Mohs 6.5–7), so Corundum will scratch Diaspore and not the other way round. It scratches glass and a steel knife and cannot be scratched by a steel file. Streak color differs: Corundum leaves a white (colorless) streak on unglazed porcelain, Diaspore leaves white. Corundum feels noticeably heavier for its size (specific gravity 3.95–4.10 versus 3.3–3.5 for Diaspore).
Which is harder, Corundum or Diaspore?
Corundum is harder: Mohs 9 against Mohs 6.5–7 for Diaspore.
Which is more valuable, Corundum or Diaspore?
Typical specimens of Corundum and Diaspore trade in a similar range, around $20.
