Crystal guide
How to Identify Spinel
MgAl₂O₄
Spinel is magnesium aluminum oxide (MgAl₂O₄) forming cubic octahedral crystals at Mohs 8. Identify it by sharp octahedra, vitreous luster, singly refractive optics, a wide red-pink-blue-violet range, and the historic confusion with ruby it has now outgrown.
About Spinel
Spinel is magnesium aluminum oxide (MgAl₂O₄) forming cubic octahedral crystals at Mohs 8. Identify it by sharp octahedra, vitreous luster, singly refractive optics, a wide red-pink-blue-violet range, and the historic confusion with ruby it has now outgrown.
Spinel is filed under crystals in the RockScan directory. Composition: Magnesium aluminum oxide. It forms via metamorphic marble and skarn environments. Typical occurrence: Myanmar, Sri Lanka, Tajikistan, Vietnam, Tanzania. For the broader method, see how to identify rock types.
Spinel properties
| Mohs hardness | 8 |
|---|---|
| Class | crystal |
| Scientific name | MgAl₂O₄ |
| Color | Red, pink, blue, violet, black |
| Luster | Vitreous |
| Crystal system / texture | Cubic |
| Composition | Magnesium aluminum oxide |
| Formation | Metamorphic marble and skarn environments |
| Occurrence | Myanmar, Sri Lanka, Tajikistan, Vietnam, Tanzania |
How to identify Spinel
- Octahedral cubic crystals; singly refractive under the loupe.
- Mohs 8 — harder than garnet-family lookalikes, softer than ruby.
- Red to cobalt-blue range; natural inclusions, never curved striae.
Lookalikes to rule out
- Ruby: Ruby is corundum (Mohs 9) with chromium fluorescence and hexagonal habit; spinel is cubic (Mohs 8), often octahedral, with a distinct spectrum.
- Garnet: Both are cubic and singly refractive, but garnet rarely forms sharp octahedra and carries a different absorption spectrum and inclusion scene.
- Synthetic spinel: Flame-fusion synthetics show curved striae and bubbles with too-perfect color; natural spinel shows crystal growth and mineral inclusions.
Photo tips for Spinel ID
- Shoot octahedral crystal faces when present — the eight-sided habit is spinel's calling card.
- Photograph red, pink, blue, and violet variants in daylight; spinel's color range is the story.
- Loupe-shot inclusions; natural spinel carries octahedral negative crystals and fingerprints, not curved striae.
Snap one specimen in natural light, then open the result in the online studio or the rock identifier app.
Spinel FAQ
How do you identify spinel?
Look for octahedral cubic crystals at Mohs 8 with single refraction. Ruby is harder with hexagonal habit; synthetics show curved striae.
Spinel vs ruby?
Ruby (corundum, 9) out-scratches spinel (8) and fluoresces differently; spinel forms octahedra while ruby favors hexagonal prisms.
Why is spinel famous?
Crown-jewel 'rubies' like the Black Prince's stone proved to be spinel — now prized in its own right, especially cobalt blue and Jedi reds.
How to identify spinel
Spinel spent centuries mistaken for ruby, so modern ID leans on habit, hardness, and inclusions rather than color:
- Octahedra: sharp eight-sided cubic crystals — ruby never grows this habit.
- Hardness 8: scratches topaz; ruby (9) scratches spinel in return.
- Single refraction: no facet doubling under the loupe, unlike peridot or zircon.
- Natural inclusions: octahedral negatives and fingerprints; curved striae betray synthetics.
- Photo confirm: octahedron macros plus daylight color shots, then RockScan against ruby and garnet.
Spinel vs ruby vs garnet vs synthetic spinel
- Ruby: harder corundum with hexagonal prisms and strong chromium fluorescence.
- Garnet: cubic too, but dodecahedral habit and a different spectrum and inclusion suite.
- Synthetic spinel: curved growth striae, bubbles, and overly even saturated color.
Photo tips for spinel habit
Octahedron faces plus a loupe inclusion shot separate natural spinel from every ruby lookalike. Compare garnet, topaz, and opal guides, then confirm in the online studio or rock identifier app.
Related crystals to compare
Browse all: rocks · minerals · crystals · directory · what rock is this · rock vs mineral vs crystal. Confirm with the online studio or rock identifier app.