Spinel
Spinel has one of the more striking stories in gemology: for centuries, it was mistaken for a different gem entirely. Before reliable testing for refractive index and specific gravity existed, red spinel and ruby were routinely confused, since the two can look nearly identical to the eye despite being chemically and structurally unrelated. The most famous example is the “Black Prince’s Ruby,” a large red stone set in the British Imperial State Crown since the 14th century and long believed to be a ruby — it’s a spinel. Several other historic “rubies” in European crown jewels have similarly turned out, on modern examination, to be spinel.
Chemically, spinel is magnesium aluminum oxide, a distinct mineral species from corundum (which is aluminum oxide, the mineral behind ruby and sapphire). Its color range is broad — vivid red and pink are the best-known, but spinel also occurs in blue, purple, and other shades — and unlike ruby’s chromium-driven red, spinel’s color comes from a different combination of trace elements depending on the hue. Myanmar, Sri Lanka, Tanzania, Tajikistan, and Vietnam are principal sources, with Myanmar’s Mogok region historically producing much of the finest red material — the same region famous for ruby, which partly explains the centuries of confusion between the two.
At Mohs 8, spinel is a genuinely durable everyday gem — harder than topaz or quartz, though softer than corundum or diamond — and well suited to rings and other jewelry worn regularly. It needs no unusual care: standard cleaning and normal precautions against hard knocks are sufficient.
Spinel’s birthstone status is the newest on this site by a wide margin. It has no place on the historical, US 1912, or UK 2013 lists at all — its addition to a monthly list came only in 2016, when the American Gem Trade Association added it to the current US list for August, alongside the long-standing peridot and sardonyx. That makes spinel a useful data point for the site’s broader argument: birthstone lists are not fixed traditions but living, occasionally-revised trade standards, and spinel’s arrival is one of the most recent and best-documented revisions on record. It doesn’t appear on either zodiac birthstone system tracked here.
Gemmological properties
| Mohs hardness | 8 |
|---|---|
| Crystal system | Cubic |
| Chemical formula | MgAl2O4 |
| Refractive index | 1.712–1.717 |
| Color family | Red, Pink, Blue, Multicolour |
| Variety of | — |
| Also known as | — |
| Origins | Myanmar, Sri Lanka, Tanzania, Tajikistan, Vietnam |
| Organic material | No |
For centuries, red spinel was mistaken for ruby in historic crown jewels — including the "Black Prince's Ruby" in the British Imperial State Crown — because the two are visually similar without gemological testing.
Where Spinel appears as a birthstone
By month
| Month | System | Confidence |
|---|---|---|
| August | US current | high |
Frequently asked questions
- Was spinel really mistaken for ruby in royal jewels?
- Yes, most famously the 'Black Prince's Ruby' set in the British Imperial State Crown — for centuries identified as a ruby, it's actually a large red spinel. Before modern gemological testing existed, the two were routinely confused because they can look very similar to the naked eye despite being chemically unrelated species.
- How is spinel different from ruby, if they can look alike?
- They're entirely different minerals. Ruby is corundum, aluminum oxide; spinel is magnesium aluminum oxide, a distinct species with its own crystal structure. Red spinel's color also comes from a different trace-element mechanism than ruby's chromium-driven red. They only resemble each other visually — testing separates them easily.
- When did spinel become an official birthstone?
- Recently — the American Gem Trade Association added spinel to the current US list for August in 2016, alongside the long-established peridot and sardonyx. It doesn't appear on the historical, US 1912, or UK 2013 lists, or on either zodiac system tracked here.