Spinel
MgAl₂O₄
COMPARE
Side by side
Put up to four minerals or gems next to each other and test them like a gemmologist.
Ruby
Al₂O₃
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.
- Spinel 3.6–4.1
- Ruby 3.98–4.1
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
- Spinel 1.719
- Ruby 1.759–1.772
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.
SpinelCubic
RubyTrigonal
PROPERTIES
All properties
| Spinel | Ruby | |
|---|---|---|
| Type | Gem | Gem |
| Mineral | Spinel | Corundum |
| Formula | MgAl₂O₄ | Al₂O₃ |
| Mineral group | Oxides | Oxides |
| Crystal system | Cubic | Trigonal |
| Mohs hardness | 7.5–8 | 9 |
| Density | 3.6–4.1 g/cm³; composition controls the range | 3.98–4.10 g/cm³ |
| Refractive index | n ≈ 1.719 for Mg-rich spinel; rises with Fe, Zn, and Cr substitution | nω about 1.767–1.772; nε about 1.759–1.763 |
| Optical class | Isotropic | Uniaxial (−) |
| Colour | Red and pink are classic; also blue, violet and other colours | Red to purplish red |
| Colour cause | Red spinel is coloured by chromium, the same trace element that colours ruby and emerald. | Chromium is the trace element behind ruby's red, which ranges from orangy red to purplish red; more chromium gives a stronger red. Chromium also makes ruby fluoresce red, which intensifies its colour. Rubies formed in marble, such as those from Myanmar, the Himalayas and northern Vietnam, contain little iron, so their red is especially intense. |
| Treatments | — | Ruby can be treated to change its colour, apparent clarity or durability. GIA research has documented low-temperature heating, flux healing of fissures, and filling of fractures with glass. |
| Look-alikes | Spinel was mistaken for ruby for centuries, and many famous historical "rubies" are spinel — among them the Black Prince's Ruby set in the front of Britain's Imperial State Crown and the Timur Ruby, which weighs over 350 carats and carries several Persian inscriptions. The two were separated scientifically only in 1783, when Jean-Baptiste Louis Romé de l'Isle distinguished them by their crystal shapes (GIA reading list). | Red spinel is coloured by the same element, chromium, and was mistaken for ruby for centuries (see Spinel). Laboratory-grown ruby, with essentially the same chemical, physical and optical properties, also exists. |
| Crystal habit | Octahedral crystals are classic; also dodecahedral, granular, or massive. Contact twins on {111} define the spinel law. | Barrel-shaped, pyramidal, tabular, or prismatic crystals; also granular and massive. Parting and growth features may be prominent. |
| Cleavage | Imperfect parting on {111}; true cleavage is indistinct | No true cleavage; common basal and rhombohedral parting |
| Fracture | Conchoidal to uneven or splintery | Uneven to conchoidal |
| Tenacity | Brittle | Brittle, but compact material is tough |
| Luster | Vitreous | Adamantine to vitreous |
| Streak | White | White |
| Transparency | Transparent to opaque | Transparent to opaque |
| Fluorescence | Usually inert, though some specimens fluoresce; trace elements can produce strong colour. | Variable; ruby may fluoresce red under ultraviolet light and some material phosphoresces. |
| Identification | Look for octahedral form, high hardness (7.5–8), lack of strong cleavage, and isotropic optics. Red spinel is singly refractive, unlike doubly refractive ruby. | 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. |
| In Tajikistan | Kuh-i-Lal in the Ishkoshim district is the historic source of gem spinel, the "balas ruby" of Badakhshan. | Gem ruby is documented from the marble-hosted Snezhnoe deposit in the Kukurt district, Eastern Pamir. |
| Source | Handbook of Mineralogy — Spinel | Handbook of Mineralogy — Corundum |