These three red gems have been confused for centuries, and the quickest ways to separate them are simple: garnet often reacts to a strong magnet, while ruby and spinel don’t; ruby glows red under UV light and shows doubled facet edges under magnification, while garnet and spinel don’t. Price differs enormously too, with fine ruby costing many times more than spinel and hundreds of times more than common red garnet. Some of history’s most famous “rubies” turned out to be spinels.
This guide compares all three from every angle. You’ll learn the practical tests, the price differences, and the famous historical mix-ups. At Gems Lore, we want you to buy with confidence.
A quick note first. This is general educational information, not a substitute for professional gemological testing.
The Quick Comparison
- Hardness: Ruby 9, spinel 8, garnet 7 to 7.5.
- Magnet: Garnet often reacts; ruby and spinel don’t.
- UV light: Ruby usually fluoresces red; garnet doesn’t; spinel varies.
- Doubling: Ruby shows doubled facet edges; garnet and spinel don’t.
- Price: Ruby is the most expensive by far, then spinel, then garnet.
Why These Three Get Confused
For most of history, “ruby” simply meant “red gemstone.” There was no way to test chemistry, so red stones of similar appearance were grouped together by color alone.
Ruby is corundum, colored by chromium. Spinel is magnesium aluminum oxide, often colored by chromium too, which is precisely why fine spinel can look so convincingly like ruby. Garnet is a family of silicates, colored mostly by iron. Three unrelated minerals that happen to share a color, and to the naked eye, a fine spinel and a fine ruby can be genuinely hard to tell apart.
The Famous Historical Mix-Ups
This confusion produced some spectacular errors, and they’re worth knowing because they show how convincing the resemblance is.
The Black Prince’s Ruby, set in the front of the British Imperial State Crown, is not a ruby at all. It’s a large red spinel, and it has been part of English royal history since the 1300s. The Timur Ruby, another celebrated stone in the Royal Collection, is likewise a spinel.
Only in the late 1700s did mineralogists establish that spinel was a distinct species from corundum. Until then, the world’s most famous rubies included stones that weren’t rubies. Garnet had its own moment too: Bohemian garnet was so prized in Victorian jewelry that it was sometimes marketed in ways that blurred the line, as our garnet history guide describes.
Test 1: The Magnet
This is garnet’s giveaway, and it’s the easiest test you can do at home. It works because of chemistry, not magic.
Garnets contain iron and manganese, which respond to magnetism. Balance the stone on a light, free-moving support, such as a small cork floating in water or a swing of thread, then bring a strong neodymium magnet close. Many garnets, especially almandine and pyrope reds, will visibly pull toward it.
Ruby and spinel contain no comparable iron content and won’t react. So a red stone that jumps toward a magnet is very likely garnet. Our real vs. fake garnet guide covers this test in more depth.
Test 2: Ultraviolet Light
UV is the ruby test. Most ruby contains chromium, which fluoresces, so it glows a distinct red under longwave UV light, sometimes strikingly.
Garnet essentially never fluoresces, because its iron content quenches the effect. Spinel is the middle case: red spinel often fluoresces red too, though usually less intensely than ruby, so this test separates garnet cleanly but doesn’t fully settle ruby versus spinel.
A curious consequence: fine Burmese ruby fluoresces so strongly it appears to glow in sunlight, which is part of what makes top ruby look so alive.
Test 3: Doubling Under Magnification
This one requires a loupe but is decisive between ruby and the other two. It comes down to optical structure.
Ruby is doubly refractive, meaning light splits in two as it passes through. Look through the table at the back facet edges under 10x magnification, and you’ll see them appear doubled, slightly blurred into two lines.
Garnet and spinel are both singly refractive, so their facet edges stay crisp and single. If you see clear doubling in a red stone, you’re looking at ruby, not spinel or garnet.
Test 4: Hardness and Wear
Hardness rarely settles an identification on the spot, but it shows over time. Ruby is 9 on the Mohs scale, second only to diamond among natural gems.
Spinel is 8, and garnet is 7 to 7.5, as our gemstone hardness chart shows. In an antique piece, this leaves traces: a decades-old garnet often shows softened, abraded facet edges, while a ruby of the same age usually stays crisp. It’s a clue rather than proof, but a useful one.
Test 5: Color Character
Color alone is unreliable, but the three do tend toward different characters. Trained eyes use this as a first impression, not a verdict.
Ruby at its best is a pure, slightly bluish red, sometimes called pigeon’s blood, and it typically has a vivid, glowing intensity. Red spinel often leans slightly orange or pink-red, and tends to look bright and clean. Garnet, especially almandine, tends darker and can look brownish or blackish in poor material, a tendency our garnet quality and grading guide explores.
The catch: fine rhodolite garnet, top spinel, and ruby can all overlap. Color narrows the field but never closes the case.
The Price Difference
This is where identification really matters, because the financial gap is enormous. These are not stones of similar value.
Fine ruby is among the most expensive gems on earth, with top unheated Burmese material reaching six figures per carat. Spinel, long undervalued, has risen sharply and now commands serious prices for fine red material, though still far below comparable ruby. Common red garnet costs roughly $10 to $50 per carat, as our garnet price guide details.
So mistaking a garnet for a ruby is a costly error, and historically, mistaking a spinel for a ruby was an imperial one.
Which Should You Buy?
Each has a genuine case, so it depends on your goal. There’s no wrong answer here.
Ruby if you want the classic, the hardest, and the most prestigious, and the budget allows. Spinel if you want ruby’s look with better value and no treatment worries, since spinel is usually untreated while ruby is routinely heated. Garnet if you want beautiful natural red on a real-world budget, with good durability and no treatment either, which our garnet buying guide covers.
For a red stone you’ll wear daily without anxiety, garnet is a genuinely smart choice. For an heirloom, ruby. For the connoisseur’s pick, spinel.
When to Get It Tested
If a stone’s identity affects its value meaningfully, get a report. A gemologist settles the question in minutes with refractive index and spectroscopy.
This matters most when buying anything sold as ruby, or inheriting an antique red stone of unknown identity. Our gemstone certification guide explains reports and when they’re worth the cost.
Frequently Asked Questions
How can you tell a ruby from a garnet?
The fastest home test is a magnet: many garnets pull toward a strong neodymium magnet, while ruby doesn’t. Under UV light, ruby usually glows red and garnet doesn’t. Under 10x magnification, ruby shows doubled facet edges, since it’s doubly refractive, while garnet’s stay crisp.
Is spinel a ruby?
No. Spinel is a completely different mineral, though the two were confused for centuries because chromium colors both. Famously, the Black Prince’s Ruby in the British Imperial State Crown is actually a red spinel, as is the Timur Ruby. Spinel was only recognized as distinct from corundum in the late 1700s.
Is garnet cheaper than ruby?
Enormously. Common red garnet costs roughly $10 to $50 per carat, while fine ruby can reach six figures per carat, and even modest ruby costs many times more than garnet. Spinel sits between them, offering ruby’s look at a fraction of the price.
Which is more durable, ruby, spinel, or garnet?
Ruby, at 9 on the Mohs scale, is the hardest, second only to diamond among natural gems. Spinel follows at 8, and garnet at 7 to 7.5. All three are durable enough for everyday jewelry, though ruby resists scratching best over decades of wear.
Is red spinel better than garnet?
Neither is objectively better. Spinel is rarer, harder, usually brighter, and considerably more expensive, making it a connoisseur’s stone. Garnet is far more affordable, equally natural and untreated, and durable enough for daily wear. For value and everyday beauty, garnet wins; for rarity, spinel.
Does garnet glow under UV light?
No. Garnet essentially never fluoresces, because its iron content quenches the effect. This makes UV a clean way to separate garnet from ruby, which usually glows red. Red spinel often fluoresces too, so UV distinguishes garnet clearly but doesn’t fully separate ruby from spinel.
Where to Go From Here
Three red stones, three different minerals, and a history of confusing them all the way up to the Crown Jewels. The tests are refreshingly simple: reach for a magnet to find garnet, a UV light and a loupe to find ruby, and remember that a stunning red stone that isn’t either might be the spinel connoisseurs prize.
Learn more in our garnet meaning and types of garnet guides, or view styles in our garnet ring guide. You can also explore more stones in our types of gemstones guide.
Have a red stone you can’t identify? Tell us about it in the comments.
This guide is for general educational purposes and is not a substitute for professional gemological testing.
