Garnet vs Ruby: How to Separate Two Red Stones

Short answer: ruby is red corundum, aluminum oxide colored by chromium, Mohs 9, and it is doubly refractive, so it shows two shades of red through a dichroscope and often glows red under longwave UV. Red garnet (pyrope or almandine) is a silicate of a different structure, Mohs 7 to 7.5, singly refractive, usually darker in tone, and normally inert under UV. In rough form the shapes differ too: ruby grows six-sided, garnet grows ball-like.

Why have so many "rubies" turned out to be garnet?

The confusion is centuries old and partly deliberate. Before gem testing existed, any transparent red stone could be called a ruby, and the trade kept the habit by attaching a place name to the word. Minerals.net lists more than ten such names that actually mean pyrope garnet, among them Cape Ruby, Arizona Ruby, Bohemian Ruby, Montana Ruby and Rocky Mountain Ruby, and gives a blunt rule: any time the word ruby is used with a prefix, outside a few accepted cases, it is a fake or a less valuable red gemstone (Minerals.net). The same list maps "Alabandine Ruby" to almandine garnet and "Balas Ruby" to spinel.

Spinel is the third stone in this story. The International Gem Society notes that the famous Black Prince's Ruby in the British crown jewels was found to be a red spinel, as were many other historic "rubies" (IGS). So when an old piece or a cheap lot is labeled ruby, three answers are possible: corundum, garnet or spinel.

What separates garnet and ruby on paper?

The figures come from webmineral's corundum, pyrope and almandine pages and the IGS gem listings. Two rows do the real work for a buyer: pleochroism and fluorescence. The density row is a trap. Almandine is as heavy as ruby, so a heft test cannot separate those two, even though it will flag a light pyrope.

Which tests can a buyer actually run?

Shine a longwave UV light on it

Chromium is the coloring agent in ruby, and GIA explains that it can also cause fluorescence. Rubies from marble deposits, which carry little iron, typically fluoresce red under ultraviolet light, even the UV in daylight (GIA). Webmineral lists almandine and pyrope as non-fluorescent. A red glow is therefore strong evidence for ruby. No glow proves less, because iron-rich rubies from basalt-related deposits can be dull too.

Look through a dichroscope or a polarizing filter

A dichroscope is an inexpensive tube that shows two windows side by side. Ruby is doubly refractive and the IGS lists its pleochroism as strong, purplish red against orangy red. Garnet is cubic: the IGS describes pyrope and almandine as generally isotropic, with no pleochroism (IGS garnet). Two different reds in the windows means corundum (or another doubly refractive stone); identical windows in every direction points to garnet, spinel or glass.

Count the faces on rough crystals

Uncut material answers the question by shape. Corundum crystals are six-sided; Geology.com shows Indian ruby segments with that outline and flat basal parting across them (Geology.com corundum). Garnets rarely show a flat hexagon. They grow as rounded 12-sided or 24-sided crystals, which is why a bag of rough pyrope looks like dark red gravel rather than tablets. The garnet guide covers those habits in detail.

Judge the tone, not just the hue

Fine red garnet can rival ruby's hue, but the IGS points out that chrome pyropes typically have a very dark tone, which distinguishes them from rubies. Hold both against white paper in daylight. Garnet reads as wine, black cherry or brownish red; ruby tends to look brighter in the center of the stone and less brown.

Try a strong magnet

Almandine and spessartine are iron- and manganese-rich, and many respond to a strong neodymium magnet when the stone is floated on a cork in water or hung on a thread. Ruby barely moves. Treat the magnet as a hint, since pyrope varies in iron content.

Skip the scratch test

Ruby is 9 and garnet 7 to 7.5, so in principle ruby scratches garnet. In practice a scratch test damages one or both stones and tells less than the four checks above. Our hardness testing guide explains when scratching is worth it.

Do garnet and ruby form in the same rocks?

Partly. Both are products of heat and pressure, but corundum needs rock that is short of silica. GIA describes the most renowned rubies, from Myanmar, the Himalayas and northern Vietnam, as growing in marble, which forms when limestone is metamorphosed. Geology.com adds that some of the most important ruby and sapphire deposits come from gems weathered out of basalt flows and concentrated in the downslope soils and sediments.

Red garnet is everywhere by comparison. GIA calls it one of the most common and widespread of gems, found in metamorphic rocks on every continent (GIA garnet). There are more than twenty garnet species, of which five are commercially important as gems. That abundance is the whole price story: garnet is common enough to be mined as an abrasive, while gem ruby is one of the most valuable colored stones.

What about ruby in fuchsite, kyanite and zoisite?

These are the affordable way to own real corundum. The ruby in them is genuine but opaque to translucent, pinkish to purplish red, set in a green or blue host: fuchsite (a chromium mica), kyanite, or zoisite in the Tanzanian rock sold as ruby zoisite. They are not garnet. If a hexagonal outline shows on the red patches of a slab, that is the corundum habit showing through.

How do treatments and prices differ?

Ruby is expected to be treated. The IGS advises assuming that most commercial rubies have undergone some form of heat treatment, which the trade accepts. Diffusion coloring and filling with polymer or lead glass are less accepted and significantly reduce value. Lead-glass-filled ruby is the material behind most suspiciously cheap transparent "rubies" in jewelry, and only a lab report settles it.

Garnet is close to the opposite. The IGS describes the "Proteus" treatment as the only regularly applied garnet treatment, so a red garnet is almost always untreated. It is also inexpensive: rough red garnet sells by the bag, and even clean faceted pyrope and almandine cost a fraction of ruby. Price is a test in itself. A clean, bright, transparent "ruby" of several carats at garnet money is either garnet, glass-filled corundum, synthetic, or glass.

Which should you buy?

• Buy ruby if you want corundum itself: the July birthstone (see the July birthstone guide), a hard stone for daily wear, or an opaque ruby-in-matrix piece for display. Ask about treatment on anything transparent.
• Buy garnet if you want deep red color on a small budget, untreated material, the January birthstone, or real crystals with sharp faces. Natural rough garnet crystals are common; natural rough ruby crystals with good color are not.
• Buy one of each if you are learning. A UV torch and a dichroscope turn the pair into a lesson that sticks.

Choosing ruby and garnet pieces we stock

When reading a listing, look for a named species (pyrope, almandine, hessonite) rather than just "red garnet", a locality, and photos that show the crystal outline. For ruby, opaque matrix pieces should be sold as ruby in a named host, never as gem ruby.

• Ruby: small natural ruby pieces from the cheap end of the corundum range, good for running the UV and dichroscope checks against a garnet.
• Ruby in Fuchsite: opaque pink-red corundum in green chromium mica, the classic display form of real ruby.
• Ruby in Kyanite: ruby crystals in blue bladed kyanite, a contrast piece where the six-sided corundum outline is often visible.
• Pyrope Garnet Rough Specimen Bag 35g: about 35 g of 1 to 3 cm rough pyrope from Madagascar, the "Cape ruby" type of garnet in its natural state.
• Red Garnet: an inexpensive single red garnet, handy as the reference stone for tone comparisons.

Frequently asked questions

Is garnet a cheap ruby?

No. Garnet is a separate mineral group with its own chemistry and cubic structure, not a lower grade of ruby. The two only share a red color. Garnet is cheaper because red garnet is common in metamorphic rocks worldwide, while gem quality corundum with strong red color is scarce.

How can I tell if my ruby is actually garnet?

Use a dichroscope and a longwave UV torch. Two different reds in the dichroscope windows and a red glow under UV point to ruby. Identical windows and no glow point to garnet or spinel. Old trade names such as Cape ruby or Arizona ruby always mean garnet.

Which is harder, garnet or ruby?

Ruby. Corundum is 9 on the Mohs scale, second only to diamond among natural gems in common use. Red garnets are 7 to 7.5. Both are hard enough for everyday jewelry, but garnet shows facet wear sooner.

Is a dark red stone more likely to be garnet?

Often, yes. Pyrope and almandine tend to be dark in tone, with brownish or purplish red that can look almost black in small stones. Ruby usually looks brighter, and fluorescence can add to its glow in daylight. Tone alone is not proof, so confirm with an optical test.

What is the difference between ruby, garnet and spinel?

Ruby is corundum, aluminum oxide. Garnet is a group of silicates. Spinel is magnesium aluminum oxide. Ruby is doubly refractive and pleochroic; garnet and spinel are both singly refractive, which is why a lab uses refractive index and spectroscopy to separate those two.

Sources

• GIA, Ruby Description: https://www.gia.edu/ruby-description
• GIA, Garnet Description: https://www.gia.edu/garnet-description
• International Gem Society, Ruby Value, Price, and Jewelry Information: https://www.gemsociety.org/article/ruby-jewelry-and-gemstone-information/
• International Gem Society, Garnet Value, Price, and Jewelry Information: https://www.gemsociety.org/article/garnet-jewelry-and-gemstone-information/
• Minerals.net, The gemstone ruby: https://www.minerals.net/gemstone/ruby_gemstone.aspx
• Geology.com, Corundum: https://geology.com/minerals/corundum.shtml
• webmineral, Corundum Mineral Data: http://webmineral.com/data/Corundum.shtml
• webmineral, Pyrope Mineral Data: http://webmineral.com/data/Pyrope.shtml
• webmineral, Almandine Mineral Data: http://webmineral.com/data/Almandine.shtml