July Birthstone: Ruby, and the Treatment Question

By Lisa Chen, Founder · July 16, 2026

Short answer: July's birthstone is ruby — and ruby is not its own mineral. It is corundum, the same species as sapphire. Red corundum is called ruby; every other color of corundum is called sapphire. That one fact explains why ruby is hard enough to wear every day, and why the treatment question matters more here than almost anywhere else in the gem trade.

What a ruby actually is

Corundum is aluminum oxide, and in its pure state it is colorless. Everything you find beautiful about a ruby is trace chemistry. Per the GIA, "chromium is the trace element that causes ruby's red," and "the more chromium, the stronger the red color."

So corundum is the species, with exactly two named varieties. Red corundum is ruby; everything else — blue, yellow, pink, green, colorless — is sapphire. The GIA puts it the other way round: sapphires "come in every color except red," and "red corundum earns the name ‘ruby.’"

Why Mohs 9 means you can actually wear it

The USGS defines hardness as "a gemstone's resistance to scratching," measured against a scale of ten minerals developed by the Austrian mineralogist F. Mohs in 1822. It lists corundum at 9 — naming ruby and sapphire as its varieties — with diamond at 10 and quartz at 7. Jewelers of America agrees: ruby is "a durable gemstone with a hardness of 9.0."

Hardness is only half of durability, though, and it is the half people over-quote. Hardness resists scratching; toughness resists breaking. Ruby is credible at both — the GIA's durability line is "Corundum (ruby and sapphire) is relatively hard and tough; a great choice for daily worn jewelry." Quartz at 7 is the number that matters to your day: household dust contains quartz, so anything softer is slowly sanded by the room it sits in. At 9, ruby outranks nearly everything it meets. That is why corundum ends up in rings worn for decades. Not sentiment. Crystallography.

One caveat the number hides, and it matters below: the GIA notes inclusions "can also limit a ruby's durability," and "significant surface-reaching fractures can pose durability threats." That everyday-wear question is worth bringing to any ring you consider — you can browse our rings collection with it in mind.

The treatment picture, stated plainly

Ruby is treated — not sometimes, but as a norm. The GIA says "most ruby has been heated, and some gems on the market also have had color diffused into the surface or fractures filled with lead glass," and puts the value question in four words worth memorizing: "You should assume your ruby is heated." That is not a scandal — but these treatments are not equivalent, and lumping them under one word is how people get hurt.

Treatment	What it does	What it means for you
Heat	Per the GIA, removes purplish coloration for a purer red, and can remove "silk" (needle-like inclusions).	The norm, and accepted — "stable to normal conditions of wear and care." Untreated stones are rare and command a premium.
Lattice diffusion	Diffuses color into the stone's surface.	Per the GIA, worth less than heated ruby.
Dyeing	Adds color by dye rather than chemistry.	Per Jewelers of America, not considered durable.
Lead-glass filling	Per the GIA, in lower-quality material, fractures and cavities "may be filled with a glass to decrease their visibility."	A different product. Legally not an unqualified "ruby" at all.

Composite ruby: the one that is genuinely a trap

Heat rearranges the stone you already have. It adds nothing, it is permanent, and what sits in your ring afterward is still one piece of corundum. That is why the trade shrugs at it.

Lead-glass filling adds a second material. It starts with corundum too fractured to sell, then floods those fractures with high-lead glass so light passes through instead of scattering off the cracks. What comes out is a corundum-and-glass composite, held together in part by the glass. The fractures did not go away. They are full.

That changes how the object survives ordinary life. GIA researchers who tested it found the filler "fairly resistant to heat exposure during jewelry repair procedures, but it reacted readily with solvents." Jewelers of America is blunter: leaded glass used "to fill pits and cracks" is among treatments "not considered durable," and its instruction is flat — "Do not use ultrasonic or steam cleaners on fractured or filled gemstones." A routine cleaning, harmless to an ordinary ruby, can damage this one.

Which is why regulators singled it out. The FTC's Jewelry Guides make it unfair or deceptive to describe gemstone material with "any amount of filler or binder, such as lead glass" using the unqualified word "ruby" — and, pointedly, "treated ruby" is not adequate either. "Composite," "hybrid," or "manufactured" may be used only when qualified to disclose that the product "does not have the same characteristics as the named stone" and "requires special care." Any amount. Not "a lot of glass" — any.

Lab-created ruby is real ruby. A simulant is not.

Lab-created ruby is corundum — the same aluminum oxide, the same chromium, the same hardness. That is a legal condition, not a marketing softener: the FTC makes it unfair or deceptive to use "laboratory-grown" or "synthetic" with a natural stone's name "unless such product has essentially the same optical, physical, and chemical properties as the stone named." A seller may say lab-created ruby precisely because it genuinely is ruby. Its far lower price is not a red flag — it is the honest consequence of skipping the geology. What it is not is mined, and the law requires that said out loud.

A simulant is a lookalike, and a lookalike is a different mineral. Red cubic zirconia, red spinel, red garnet, and red glass are all red stones that are not corundum. CZ and moissanite are simulants — and they are not diamonds either, whatever a listing implies. The FTC requires the distinction be visible rather than mumbled: "laboratory-grown," "imitation," or "simulated" must immediately precede the stone's name "with equal conspicuousness," and the Guides note "faux" does not count.

Practically: red does not mean ruby. In inexpensive jewelry a red stone is far more likely to be cubic zirconia, glass, or lab-grown corundum than a mined gem — which is fine, so long as the listing says which. What should give you pause is a listing that says "ruby" and goes quiet.

Where July's claim to ruby comes from

The modern US birthstone list is a trade convention, not an ancient inheritance. Jewelers of America, the self-described originator of the US birthstone jewelry list, dates it to 1912, "when it was established by the American National Retail Jewelers Association, which has become today's Jewelers of America." July's entry never needed an addition: ruby, alone.

Ruby also carries a long train of symbolism, and the honest way to hand it over is as history. The GIA records that ancient India called ruby the "king of precious stones," that Burmese warriors believed it made them invincible, and that medieval Europeans held it bestowed health, wisdom, wealth and success in love. That is culture, and it is genuinely interesting. It is not a claim that a ruby will do anything for your health, wealth, luck, relationships, or character. It is aluminum oxide with a little chromium in it. The meaning is the part we brought.

Frequently asked questions

Is ruby the same thing as sapphire?

Almost. Both are the mineral corundum — the GIA describes ruby as "one variety of the corundum mineral species, which also includes sapphire." Red corundum is ruby; corundum of every other color, including colorless, is sapphire. The red comes from chromium.

How hard is ruby, and can I wear it every day?

The USGS lists corundum, the mineral of both ruby and sapphire, at 9 on the Mohs scale of scratch resistance, with diamond at 10 and quartz at 7. Jewelers of America gives ruby a hardness of 9.0. Hardness is only part of durability, but the GIA's verdict covers both halves: corundum is "relatively hard and tough; a great choice for daily worn jewelry."

Are most rubies treated?

Yes. The GIA states that most ruby has been heated, and advises: "You should assume your ruby is heated." Heat treatment is accepted because it is stable to normal conditions of wear and care. Lattice diffusion, dyeing, and lead-glass filling are different matters, and the GIA notes diffused or glass-filled rubies are worth less than heated ones. Always ask which applies, in writing.

What is a lead-glass-filled or composite ruby?

It is low-quality corundum whose fractures have been filled with glass so the stone looks more transparent — a composite rather than a solid gem, and less durable. GIA researchers found the filler reacted readily with solvents, and both the GIA and Jewelers of America say fractured or filled stones must never go in ultrasonic or steam cleaners. The FTC's Jewelry Guides state it is unfair or deceptive to describe gemstone material containing any amount of filler such as lead glass with the unqualified word "ruby," or as a "treated ruby."

Is lab-created ruby real ruby, and how is it different from a simulant?

Lab-created ruby is real ruby — corundum with essentially the same optical, physical, and chemical properties as a mined stone, which the FTC requires before that name may be used. It is legitimately cheaper because it is not mined, and that must be disclosed. A simulant is different: red cubic zirconia, red glass, or red spinel only resemble ruby. The FTC requires "laboratory-grown," "imitation," or "simulated" to immediately precede the stone's name.

Ruby is corundum, hard enough for a life you actually live, and the questions that matter are what the stone is and how it was treated. For more, see our guides on September's birthstone, sapphire and August's birthstone lineup, or browse our rings collection.

If you want to wear July’s stone rather than read about it, our Personalized Cross & Birthstone Necklace is made in a July version — 18k gold plating over a 316L stainless steel base, built for daily wear. The stones are cubic zirconia, not mined gems, and the product page says so.