Sapphire
Al₂O₃
COMPARE
Side by side
Put up to four minerals or gems next to each other and test them like a gemmologist.
Diamond
C
VERDICT
How to tell them apart
THE TESTS
Gem-testing instruments
Scratch test
Mohs hardness: a mineral scratches anything softer than itself.
Heavy liquid
Density in g/cm³: denser stones sink deeper.
- Sapphire 3.98–4.1
- Diamond 3.52
Refractometer
Refractive index: how strongly the stone bends light. See light bend through a 3D crystal →
1.401.451.501.551.601.651.701.751.80
- Sapphire 1.759–1.772
- DiamondOTL 2.417
Polariscope
Between crossed polarisers a doubly refractive stone blinks as it turns; a singly refractive one stays dark. See light bend through a 3D crystal →
Crystal structures
Unit cells side by side. Drag any of them: they all turn together.
SapphireTrigonal
DiamondCubic
PROPERTIES
All properties
| Sapphire | Diamond | |
|---|---|---|
| Type | Gem | Gem |
| Mineral | Corundum | Diamond |
| Formula | Al₂O₃ | C |
| Mineral group | Oxides | Native elements |
| Crystal system | Trigonal | Cubic |
| Mohs hardness | 9 | 10 |
| Density | 3.98–4.10 g/cm³ | 3.52 g/cm³ |
| Refractive index | nω about 1.767–1.772; nε about 1.759–1.763 | n 2.417 |
| Optical class | Uniaxial (−) | Isotropic |
| Colour | Blue and every non-red gem colour | Colourless and fancy colours |
| Colour cause | Traces of iron and titanium — only a few hundredths of a percent — turn corundum blue; more iron gives a darker blue. Chromium produces pink sapphire. Sapphire also occurs in violet, green, yellow, orange, purple, grey, black and brown. | — |
| Look-alikes | Colourless sapphire was once a popular diamond imitation. Laboratory-grown sapphire is durable enough to be used for supermarket-scanner and spacecraft windows. | Colourless sapphire was once a popular diamond imitation (see Sapphire). |
| Crystal habit | Barrel-shaped, pyramidal, tabular, or prismatic crystals; also granular and massive. Parting and growth features may be prominent. | Most commonly octahedral, dodecahedral or cubic crystals; faces may show triangular growth or dissolution features. |
| Cleavage | No true cleavage; common basal and rhombohedral parting | Perfect on {111} |
| Fracture | Uneven to conchoidal | Conchoidal |
| Tenacity | Brittle, but compact material is tough | Brittle despite exceptional hardness |
| Luster | Adamantine to vitreous | Adamantine |
| Streak | White | White |
| Transparency | Transparent to opaque | Transparent to opaque |
| Fluorescence | Variable; ruby may fluoresce red under ultraviolet light and some material phosphoresces. | Fluorescence varies; blue is common, but response and strength are not diagnostic by themselves. |
| Identification | Its hardness of 9 is decisive: it scratches nearly every common mineral. High density and parting help distinguish it; ruby and sapphire are colour varieties of corundum, not separate mineral species. | Mohs hardness 10, adamantine luster, perfect octahedral cleavage and very high thermal conductivity are characteristic; reliable identification requires gemological testing. |
| In Tajikistan | Sapphire is documented together with ruby at the Snezhnoe deposit in the Kukurt district, Eastern Pamir. | — |
| Source | Handbook of Mineralogy — Corundum | Handbook of Mineralogy — Diamond |