Spinel: The August Birthstone That Was Called Ruby

What spinel is: the stone with eight faces

Spinel is magnesium aluminum oxide, MgAl₂O₄, and it crystallizes in the cubic system. You can see it in the crystals: they are nearly always octahedra, two four-sided pyramids joined base to base, often so sharp-edged they look as if someone had already cut them. The name probably comes from the Latin spina, thorn, after exactly those points. Pure spinel is colorless; everything that colors it comes from traces of other metals. Chromium makes it red and pink, iron blue to gray, cobalt a vivid blue, and vanadium together with chromium produces orange.

With a Mohs hardness of 8 it is one of the hardest gemstones there is; only corundum (ruby and sapphire) and diamond rank above it. It has no cleavage, tolerates heat and ultrasonic cleaning, and jewelers who know it love it for exactly that reason. That so few people know it is the strange part of its story.

Five hundred years as a "ruby": balas, the Black Prince and Timur

Until the eighteenth century every red stone from the East was a ruby. Traders distinguished only by origin and tone. The ruby from Burma was the "oriental" ruby; the one from Badakhshan, in today's Tajikistan and Afghanistan, was the balas ruby, after the region's old name, Balakhshan. From around the tenth century the mines of Kuh-i-Lal on the Panj River supplied the large, clean reds that traveled into the treasuries of Delhi, Isfahan, Constantinople and London, and nearly all of them were spinels.

The most famous is the Black Prince's Ruby: about 170 carats, never faceted, only polished, with a drill hole at the top that was later plugged with a small ruby. According to the traditional story it passed in 1367 from King Pedro of Castile to Edward, the Black Prince, was worn on Henry V's helmet at Agincourt, and today sits at the front of the Imperial State Crown. The Timur Ruby, 361 carats, carries engraved names of Mughal emperors and is also in the British Royal Collection; the great red stone crowning Catherine the Great's crown, about 398 carats, is a spinel from the same region. Only in 1783 did the French mineralogist Jean-Baptiste Romé de l'Isle show that spinel and ruby are different minerals. Too late for the names: they still stand that way in the inventories.

Colors and what causes them

Red is the classic color: chromium, as in ruby, usually a little lighter and without ruby's faint bluish tint. Pink and hot pink have been the most sought-after colors since 2007, when a find of neon-bright pink and red spinels at Mahenge in Tanzania reinvented the stone on the market. Orange, sometimes sold as "flame spinel," comes from Myanmar and Sri Lanka. Blue comes in two kinds worth telling apart: the grayish, steely iron blue, which is common, and the pure, glowing cobalt blue from Luc Yen in Vietnam, which is rare and expensive. Violet, lavender and gray are the quiet colors jewelers love. Black spinel, called pleonaste or ceylonite, is iron-rich and opaque, and it is the only spinel you are likely to find as a tumbled stone in a crystal shop.

Spinel and its look-alikes: the numbers that make the difference

Stone	Hardness	Refractive index	Double refraction	Crystal habit
Spinel	8	1.718	none (singly refractive)	octahedron
Ruby / sapphire	9	1.76 to 1.77	yes, weak	six-sided barrels and plates
Garnet (pyrope, rhodolite)	7 to 7.5	1.74 to 1.76	none (singly refractive)	rhombic dodecahedron
Synthetic spinel	8	1.727	none, but strain patterns	boule, no natural faces
Red glass	5 to 6	1.5 to 1.7	none	bubbles, swirls, rounded edges

How to tell spinel from ruby, garnet and lab-grown stones

The good news first: spinel is one of the few stones where a loupe and some patience can already rule out a lot. The bad news: the final proof comes from a refractometer, and that sits with a gemologist, not in your drawer.

1. Rough stone: look at the shape. A red octahedron with sharp edges is very likely spinel. Ruby grows in six-sided barrels and plates, garnet in twelve-faced crystals with rhombic faces. Fine spinel octahedra are often not cut at all, because they are beautiful as grown; many are set just as they came out of the ground.
2. Cut stone: look for doubling. Hold a 10x loupe over a back facet and look through the stone. In ruby and sapphire the back facet edges appear slightly doubled, because corundum is doubly refractive. Spinel and garnet show single crisp edges. With practice this is visible with a loupe and reliably separates spinel from corundum.
3. Read the inclusions. Natural spinel often contains tiny octahedral inclusions, "spinel in spinel," sometimes lined up like a string of pearls. Synthetic spinel, flame-fusion grown since the early twentieth century, shows curved growth lines and gas bubbles, and between crossed polarizing filters it displays a striped or crosshatched strain pattern gemologists call "tabby extinction." Natural spinel stays evenly dark.
4. UV light. Red and pink spinel usually glows a strong red under long-wave UV, much like ruby. Garnet never does. That separates spinel from garnet, but not from ruby; for that you need step 2.
5. The price tag. A "ruby" at a fraction of the price of ruby is either glass, a lab stone or, at best, spinel. Ask. A good dealer will tell you; a very good one has already written it on the label.

Spinel in America: class rings and the 2016 birthstone

If spinel sounds like a substitute to many American ears, there is a reason. For much of the twentieth century, lab-grown spinel was one of the workhorse stones of the American jewelry trade: it stood in for aquamarine, blue zircon and other colors in birthstone rings and high school class rings, where it was often simply called "birthstone" and nobody asked what it really was. So the name became associated with cheap imitation, while the natural stone was sitting in royal crowns.

That changed in 2016, when the American Gem Trade Association and Jewelers of America added spinel to the official US birthstone list as a third stone for August, next to peridot and sardonyx. It is a trade decision, not an ancient tradition, but it put natural spinel in front of American buyers for the first time in generations. Our birthstones by month guide explains how the list came to be and who has changed it.

Where spinel comes from today

The old mines of Kuh-i-Lal in Tajikistan still produce, but little. The main sources today are Myanmar (Mogok: the classic reds, often as crystals in white marble), Sri Lanka (blue, violet and gray from river gravels), Tanzania (Mahenge and Tunduru: the glowing pinks and reds), Vietnam (Luc Yen: cobalt blue and lavender), plus Madagascar and Afghanistan. When you buy a spinel with an origin on the label, you are often buying a color along with it: "Mahenge" means neon pink, "Luc Yen" means cobalt, "Mogok" means classic red.

What crystal lore attributes to spinel

Crystal lore discovered spinel late. It was simply too rare and too expensive for crystal bowls, and the black pleonaste that did make it in has a different character from the red crystal. In modern crystal books spinel is described as a stone of renewal: it is said to restore drive, lift you after exhaustion, and support ambition and stamina. Red spinel is credited with vitality, blue with calm, black with protection and grounding. I write that as plainly as it should be meant: these are attributions that suit a stone that lived under someone else's name for five hundred years and still sat in every crown. A stone for people who are underestimated and know it. What a magnesium aluminum oxide does to your body cannot be measured; what the image of an octahedron in your hand does to your posture, perhaps a little more. For how these attributions compare across stones, see our crystal meanings chart.

Care and wear

Spinel is an everyday stone without restrictions. Hardness 8, no cleavage, no sensitivity to water, soap, sweat or ultrasonic cleaning. It works in rings worn daily and does not fade in sunlight. The only caution concerns treated material: some blue and red spinels are now heated to clarify the color. That is stable, but it should be disclosed. If you own a rough crystal, keep its edges away from harder stones; an octahedron with a chipped point is no longer an octahedron. Our Mohs hardness guide shows which stones can share a pouch.

Quiz: which spinel fits you?

From the practice

From jewelers I hear the same sentence about spinel again and again: "If the customer only knew what she was getting for the price." Hardness 8, no cleavage, colors that match ruby and sapphire, and a price that, except for the top reds and the cobalt blues, still sits far below them. The stone suffers from its name, and in America from a century of class rings.

A collector who specializes in rough crystals told me she buys spinel almost only uncut: "An octahedron is already finished. Cutting it makes it smaller, not prettier." And from the crystal trade comes a sobering observation: what sits in shops as "spinel" is nearly always black pleonaste, an honest, inexpensive stone, but a different character from the crystal from Mogok. If you mean the red one, you have to look for it at a gem dealer. For the related red stones, read on with garnet.

What we do not promise

We do not claim that spinel gives strength, cures exhaustion or changes anything in your body. Crystal lore attributes that to it; it is not demonstrated, and a mineral made of magnesium, aluminum and oxygen has no mechanism to do it. We also do not claim that every spinel on the market is natural, since lab-grown spinel has been sold for a century, or that you can always separate it from ruby without a refractometer. What we do say: it is one of the hardest, most beautiful and most underrated stones you can wear. That is enough.

In practice: step by step

1. Buy it as a crystal if you can. A natural octahedron is the best proof of authenticity there is, and prettier than most cuts.
2. Ask about origin and treatment. "Mahenge," "Luc Yen" and "Mogok" are color promises; "heated" is a required disclosure. A seller who names both knows what they are selling.
3. Check for doubling. Loupe, back facet, look through: single edge means spinel or garnet, doubled edge means corundum. That is the one move worth learning.
4. Take the black pleonaste seriously. It is a good, hard, honest stone for anyone who wants grounding with a shine.
5. Wear it without worry. Hardness 8, no fading, no splitting. Our crystals and stones collection shows how stones of that hardness hold up in daily life.

Frequently asked questions

Is spinel a real gemstone?

Yes. Spinel (MgAl₂O₄) is a natural mineral with a Mohs hardness of 8 that has sat in royal treasuries since the Middle Ages, usually under the name ruby. Its reputation as a substitute comes from lab-grown spinel, widely used in American birthstone and class rings in the twentieth century.

How do you tell spinel from ruby?

Ruby is doubly refractive, so under a loupe its back facet edges look slightly doubled; spinel is singly refractive and shows single crisp edges. As rough crystals, spinel forms octahedra while ruby forms six-sided barrels. A refractometer separates them for certain (spinel 1.718, ruby 1.76 to 1.77).

What colors does spinel come in?

Red and pink (chromium), orange, blue (grayish from iron or vivid from cobalt), violet, lavender, gray and black (pleonaste). Colorless spinel is rare. The most prized are neon pink from Mahenge, Tanzania, and cobalt blue from Luc Yen, Vietnam.

Is spinel an August birthstone?

Yes. Since 2016 spinel has been on the official US birthstone list as a third August stone, next to peridot and sardonyx, a decision by the American Gem Trade Association and Jewelers of America rather than an old tradition.

What is spinel said to do spiritually?

Crystal lore calls it a stone of renewal: drive, stamina, getting back up after exhaustion; red for vitality, blue for calm, black for grounding and protection. Those are traditional attributions, not demonstrated effects.

How do you care for spinel?

Warm water, mild soap and a soft brush; ultrasonic and steam cleaning are fine. It does not fade and has no cleavage. Only protect rough crystals from knocks against harder stones.