Spinel is a singly refractive gem mineral of the cubic crystal system. Traces of chromium, iron or cobalt cause its colors.
Properties
| Refractive index | 1.718 (GIA); by color 1.712 to 1.747 (IGS); Fe-rich ceylonite/pleonaste up to 1.780 |
|---|---|
| Birefringence | None (singly refractive) |
| Optic character | Singly refractive |
| Dispersion | 0.02 |
| Specific gravity | 3.60 (GIA); gem spinel 3.54 to 3.70 (gemdat), mostly 3.58 to 3.61 (LibreTexts); ceylonite/pleonaste 3.63 to 3.90 (IGS) |
| Hardness (Mohs) | 8 |
| Crystal system | Cubic |
| Pleochroism | None |
| Absorption spectrum | Sharp line in red and pink spinel (with strong red fluorescence), 685.5 nm; sharp line in red and pink spinel (with strong red fluorescence), 684 nm; weak band in red and pink spinel, 656 nm; broad band in red and pink spinel, 540 nm (Cr); main iron line in blue spinel, 458 nm (Fe); narrow line in blue spinel, 478 nm (Fe); natural cobalt-blue spinel; not seen in synthetic material; especially diagnostic, 460 nm (Co) |
Published values. They change a little from stone to stone. Our tools do not model spinel yet. Sources: GIA · International Gem Society · LibreTexts Gemology · Gemdat
In more detail
Chromium gives reds and pinks, iron gives violet to blue, and cobalt gives vibrant blues. For centuries, people confused spinel with ruby and sapphire, so some famous "rubies" are actually spinel. Synthetic spinel is widely used as an imitation of many other gems. Some spinel is heat treated, and spinel has good toughness.
On a grading report
For cobalt spinel, LMHC wording is Species: (natural) spinel. Variety: (cobalt) spinel. It applies only when cobalt, not a treatment, mainly causes the blue.
Related terms
- Singly refractive: A singly refractive stone has one refractive index in every direction, so light stays one ray inside it. Diamond and cubic zirconia are singly refractive.
- Ruby: Ruby is the red variety of the mineral corundum. Chromium causes its red color.
- Crystal system: A crystal system groups crystals by the symmetry of their atomic structure. Gemologists use seven: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic and triclinic.
- Heat treatment: Heat treatment is controlled heating of a gem to change its color or clarity.
- Synthetic gemstone: A synthetic gemstone is made in a laboratory and has essentially the same chemistry and properties as the natural gem. A synthetic ruby is ruby, made by people.
- Simulant: A simulant is a material that looks like a gem but has a different chemistry, structure and properties. Cubic zirconia, moissanite and glass are diamond simulants.