Spinel
The Spinel Group of minerals includes Spinel, Gahnite, Galaxite, Hercynite and Magnesiochromite. Although Spinel is most famous for its red variety, it is also found in other colors such as green, blue, purple, brown and black. The blue Spinel is colored by cobalt and is quite beautiful. Star Spinels and color-change Spinels are also (rarely) found.
Spinel is a historically important gem because it has been mistaken for Ruby in some very famous crown jewel collections. Perhaps the most famous of these is the Black Princes’s Ruby set in the British Imperial State Crown (Crown Jewels of England). It is estimated to be 170 carats. Originally thought to be a large Ruby, it was later proven to be a fine Spinel. Although Spinel gems are usually small, 3 carats or less, very large gems have been cut. The largest is probably the 520 carat gem found at the Banque Markazi in Teheran, Iran.
| Category: | Oxide minerals (Spinel group) |
| Formula: | MgAl2O4 |
| magnesium aluminium | |
| Crystallography: | Isometric – Hexoctahedral |
| Crystal Habit: | Commonly as octahedra, to 30 cm, may show dodecahedron or cube faces, rarely as dodecahedra; coarse granular, rounded, massive. |
| Twinning: | On [111] as both twin and composition plane, the spinel law, penetration or contact, may be repeated as sixlings. |
| Cleavage: | None observed; Parting on [111] |
| Fracture: | Conchoidal, Irregular/Uneven to Splintery |
| Tenacity: | Brittle |
| Hardness (Mohs): | 7.5 – 8.0 |
| Density: | 3.58 – 4.10 (g/cm3) (increasing with Fe and Zn content) |
| Luminescence: | Reds and Pinks: crimson in LW UV and SW UV, red in X-rays, no phosphorescence. Blue: inert in UV. Deep Purple: red in LW UV, essentially inert in SW UV, lilac in X-rays. Pale Blue and Violet: green in LW UV and X-rays, essentially inert in SW UV. |
| Radioactivity: | Not Radioactive |
| Color: | Colorless, Red, Blue, Green, Brown, Black |
| Transparency: | Transparent to Translucent to Opaque |
| Luster: | Vitreous |
| Refractive Index: | 1.72 – 1.80 Isotropic |
| Birefringence: | 0.000 – Isotropic minerals have no birefringence |
| Dispersion: | 0.020 |
| Pleochroism: | Usually None; anomalous in some blue zincian varieties. |
