Spinel
Spinel is a gemstone belonging to the spinel group, represented by magnesia spinel (MgAl₂O₄). The name is derived from the Latin word "Spina," meaning "thorn." Historically, polished red stones were long confused with corundum such as ruby, and it is well known that the famous "Black Prince's Ruby" is actually a spinel. They come in a variety of colors, including red, pink, blue, light blue, and purplish-red, and rare colors such as colorless and cobalt blue also exist.
A vibrant red spinel rivaling ruby
Basic Information
| English Name | Spinel |
|---|---|
| Japanese Name | Senshoseki (Magnesia Spinel) |
| Chemical Composition | MgAl₂O₄ |
| Crystal System | Isometric |
| Hardness | 7.5–8 |
| Specific Gravity | Approx. 3.85 |
Characteristics, Varieties, and Trade Names
Spinel is also a group name consisting of over 20 species, of which magnesia spinel is the primary type used as a gemstone. Common colors include red, pink, blue, and purplish-red; bright green is the only color that has not been confirmed.
- Star Spinel: Commonly seen in dull purplish-red stones, these display a 4- or 6-rayed star effect (asterism). The cause of this star is believed to be needle-like inclusions derived from sphene.
- Cobalt Spinel: A variety containing Co that exhibits a vivid blue. However, it is considered difficult to draw a clear distinction from iron-colored blue spinel.
- Gahnospinel: A zinc-rich magnesia spinel ranging from blue-green to dark, blackish green, known from Sri Lanka.
- Trade Names: Red stones have been called "Balas ruby" or "ruby spinel," purplish-red stones "almandine spinel," and blue stones "sapphire spinel," named after ruby and sapphire.
Treatment, Synthesis, and Simulants
Spinel is sometimes heat-treated to reduce the presence of minor secondary colors.
Representative simulants include corundum such as ruby and sapphire, which share similar color ranges. Corundum exhibits birefringence, while spinel is isotropic; observing light-dark changes using a polarizing filter can be a clue for identification.
Many synthetic spinels produced by the flame-fusion (Verneuil) method or flux method exist, and synthetic stones nearly indistinguishable from natural stones are in circulation. It is also important to note that many stones sold as "black spinel" are actually hercynite or a mixed crystal thereof.
Handling Tips: It is difficult to distinguish spinel from ruby or sapphire based solely on color or appearance, and high-quality synthetic spinels also exist. If you are concerned about treatments or synthesis, it is safer to choose a stone that comes with an identification report.