Oxides
Spinel
Spinel carries one of gemology's most fascinating cases of mistaken identity: for centuries, red spinel was sold and worn as ruby, and several of history's most famous 'rubies' — including the Black Prince's Ruby in the British Imperial State Crown and the Timur Ruby — have since been identified as spinel instead.
The geology — what Spinel actually is
- Mineral class
- Oxide (spinel group)
- Chemical formula
- MgAl2O4
- Crystal system
- Cubic (isometric)
- Mohs hardness
- 7.5–8
What causes the color: Trace chromium produces the vivid red color that led to the historical ruby confusion, while trace iron and cobalt produce spinel's blue varieties — different trace elements from the chromium (red) and iron/titanium (blue) that color true corundum, even though the visual result can look remarkably similar.
How it forms: Forms in metamorphosed limestone (marble) and in some igneous rocks, often alongside corundum (ruby and sapphire) in the same gem gravels, which is precisely why the two got mixed up historically — they're frequently found together in the same alluvial deposits.
- Mogok, Myanmar (a historic source producing both ruby and spinel from the same region)
- Sri Lanka
- Tanzania
- Vietnam
Treatments & imitations: Unlike ruby, spinel is very rarely heat-treated — most spinel on the market is natural and untreated, which is a genuine point in its favor for buyers wary of undisclosed treatment in other colored gems.
Real vs. fake: Spinel's cubic crystal system means it forms octahedral (double-pyramid) crystals, distinct from corundum's hexagonal habit; a refractive index test easily separates the two in a gem lab, and spinel's single refraction (versus corundum's double refraction) is the definitive gemological check.
Natural, untreated, and hard enough to wear without thinking
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- Faceted natural spinel
This page makes an unusual claim for a coloured gem: most material on the market is untreated. Combined with Mohs 7.5 to 8 and a glassy lustre that needs no clarity enhancement, that puts it in genuine daily-wear territory.
- Soft brush for cleaning a faceted setting
The page's own routine for this stone is warm water and a soft brush, which works because at Mohs 7.5 to 8 the surface is harder than the household dust doing the scratching. Softer stones on this site cannot take the same treatment.
The tradition — how people use Spinel
Historical use: Spinel's documented history is largely the history of it being mistaken for ruby — the Black Prince's Ruby (in the British Crown Jewels since at least the 14th century) and the 361-carat Timur Ruby (Mughal and later British royal collections) are both now confirmed as spinel through modern gemological testing, not ruby as historically believed.
Metaphysical tradition: Modern crystal-healing tradition treats red spinel similarly to ruby — vitality, courage, passion — a natural borrowing given the centuries the two stones were literally treated as the same gem by royal courts and gem traders alike.
How to use it: Most spinel is cut into brilliant or step facets to show off its natural glassy luster, since it needs no clarity-enhancing treatment the way many other colored gems do; a well-cut spinel ring or pendant is essentially a maintenance-free daily piece.
Cleansing & care: Its Mohs 7.5–8 hardness puts it firmly in the durable-daily-jewelry category — warm water and a soft brush handle routine dust and skin oils without any risk to the stone.
Frequently asked questions
Why was spinel mistaken for ruby for so long?
Spinel and ruby form in the same geological settings and are often found together in the same gem gravels, and untreated red spinel can look remarkably similar to ruby to the naked eye. It took modern gemological testing (refractive index, crystal habit) to correctly identify famous stones like the Black Prince's Ruby as spinel.
Related crystals
Ruby
Corundum Group
Ruby and sapphire are, mineralogically, the exact same species — corundum — distinguished purely by which trace element got trapped inside during formation. Chromium turns corundum red, and red corundum is called ruby; any other trace element turns it some other color, and that's called sapphire instead. At Mohs 9, ruby is second in hardness only to diamond among gemstones, and its red color has made it, alongside sapphire and emerald, one of the traditional 'big three' precious colored gems for centuries.
Garnet
Garnet Group
'Garnet' isn't one mineral — it's a group of several closely related minerals that all share the same isometric crystal structure but differ in exact chemistry, which is why garnets come in almost every color except blue, from the deep red almandine most people picture to vivid green tsavorite and orange spessartine. Almandine, the most common variety in jewelry, gets its name from the Latin place name for the region of Turkey once associated with fine garnet, and the mineral's own name comes from the Latin for pomegranate, for its resemblance to the fruit's seeds.
Sapphire
Corundum Group
Sapphire is corundum in essentially any color other than red — blue is the best known, but pink, yellow, green, and colorless sapphire are all the same mineral species as ruby, just with different trace elements producing different colors. At Mohs 9, it shares ruby's exceptional hardness, and it has one of the longest continuously-documented gem-trading histories on Earth, with Sri Lankan sapphire changing hands for well over 2,000 years.
Sources and factual basis for the geology above: see our methodology.