Ruby is the red variety of corundum, an aluminum oxide that gets its color from small amounts of chromium. It ranks 9 on the Mohs scale, harder than any other gem in common use except diamond, so it keeps a sharp polish in rings worn every day. Fine rubies with strong, pure red color and few visible inclusions are among the most expensive colored stones per carat, and large clean ones are rare.
What ruby is made of
Corundum is aluminum oxide (Al2O3). Pure corundum is colorless. When chromium replaces a fraction of a percent of the aluminum in the crystal, the stone absorbs yellow-green and violet light and lets red and some blue pass through. Every other color of gem corundum is sold as sapphire, so ruby and sapphire are one mineral species with different trace elements.
Chromium also makes ruby fluoresce red under ultraviolet light, and ordinary daylight carries enough short-wavelength light to trigger some of that glow. Rubies low in iron, such as many from marble deposits, fluoresce strongly and look brighter in daylight. Iron dampens fluorescence, which is why rubies from iron-rich deposits linked to basalt often look darker and more subdued. The page on color in gemstones covers chromium, iron and other coloring elements.
Where pink sapphire ends and ruby begins is a matter of trade custom. Labs and markets draw the line at different levels of saturation, and a light stone sold as ruby in one country may be called pink sapphire in another.
Pigeon's blood and other color terms
The trade prizes a pure, strongly saturated red to slightly purplish red in a medium to medium-dark tone. "Pigeon's blood" is an old Burmese trade term for this top color, usually described as red with a faint blue undertone and strong fluorescence. Some laboratories now print "pigeon's blood" as a color description on reports when a stone meets their own criteria, but the term has no single universal standard. Stones that are too dark lose brightness, and stones with orange or brown modifiers sell for less.
Physical and optical properties
Ruby crystallizes in the trigonal system, often as flat hexagonal tablets or barrel-shaped prisms. It is doubly refractive, with a refractive index of about 1.762-1.770, and it shows dichroism: viewed along one direction it looks purplish red, and along another it looks orangy red. Cutters orient the table to show the better color when the rough allows it.
Corundum has no true cleavage, but it can part along planes of weakness caused by twinning. Its toughness is good, so a ruby resists chipping better than most colored gems. Specific gravity sits near 4.00, which makes a ruby feel heavier in the hand than glass of the same size. The Mohs hardness scale shows how 9 compares with other gems.
Inclusions and star rubies
Most rubies contain inclusions, and GIA places ruby in its Type II clarity group, where some inclusions are expected even in good stones. Common ones include fine rutile needles called silk, crystals of other minerals, fingerprint-like healed fractures and color zoning. Light silk spreads light across the face of the stone and softens the color, which many buyers like. Dense, aligned silk in a stone cut as a cabochon produces a six-rayed star, an effect called asterism.
Where rubies come from
Myanmar's Mogok Stone Tract has produced rubies for centuries and set the trade's benchmark for color. Mong Hsu, also in Myanmar, produced large quantities from the early 1990s, mostly stones that needed heating to remove bluish areas.
Thailand and Cambodia supplied much of the world's ruby during the 20th century from gem gravels around Chanthaburi, Trat and Pailin. These stones tend to be darker and more garnet-like in color because of their iron content. The page on gems of Thailand covers that region and Bangkok's role as a cutting and heating center.
In East Africa, the Montepuez area of northern Mozambique was discovered in 2009 and became a major source of fine ruby within a few years. Tanzania, Kenya and Madagascar also produce ruby, and Sri Lanka yields lighter, often pinkish stones from its gem gravels. Smaller sources include Afghanistan, Vietnam, Tajikistan and Greenland.
Ruby forms mainly in two geological settings. Marble-hosted deposits form when impure limestone is changed by heat and pressure, and they give the low-iron, fluorescent material associated with Mogok. Deposits linked to amphibolite and basalt give stones richer in iron. The page on metamorphic rocks and gemstones explains how marble deposits form.
Treatments
Most rubies on the market have been heated. Heating at high temperature can dissolve silk, reduce purple or blue tints and improve clarity. It is stable and accepted in the trade when disclosed, though unheated rubies of the same quality sell for more.
Other treatments are less benign. Fractures can be healed with flux during heating, which leaves glassy residue. Heavily fractured, low-grade corundum can be filled with high-lead glass, which hides cracks and improves transparency. These "composite" or "lead glass filled" rubies are worth a small fraction of untreated stones and can be damaged by heat and mild acids. Diffusion, dyeing and oiling also occur. The ruby treatments guide explains each method and how labs detect it, and the overview of gemstone treatments compares ruby with other gems.
Synthetic rubies and imitations
Ruby was the first gem made in a laboratory on a commercial scale. Auguste Verneuil announced his flame fusion process in 1902, and flame fusion synthetic ruby has been cheap and plentiful ever since. It often shows curved growth lines and gas bubbles under magnification. Flux-grown and hydrothermal synthetics followed later in the 20th century, and these can be much harder to separate from natural stones, sometimes requiring advanced lab testing.
Red spinel and red garnet were long confused with ruby. Many historic "rubies" in royal collections turned out to be spinel, including the Black Prince's Ruby in the British Imperial State Crown. Red glass and doublets with a thin garnet top fused to a glass base are common imitations.
What affects ruby value
- Color carries the most weight. A pure, saturated red in a medium to medium-dark tone sits at the top.
- Clarity matters, but eye-clean is the realistic goal. Inclusions that weaken the stone or break up the color lower value.
- Price climbs faster than weight, because fine rubies over a few carats are rare.
- Cut quality affects brightness. Many rubies are cut near the mine to save weight, which can leave windows or deep, heavy pavilions.
- Treatment status changes value sharply. Unheated, heated, flux-healed and glass-filled stones belong in separate price categories.
- Origin can add a premium, with Burmese stones traditionally commanding the most. Claims of origin need a report from an established lab.
For a wider checklist, see how to buy a gemstone and how colored gemstones are graded.
Caring for ruby
Untreated and heated rubies are tough enough for any type of jewelry. Clean them with warm soapy water and a soft brush. Ultrasonic and steam cleaners are usually safe for these stones unless they have large fractures. Glass-filled and dyed rubies need gentle care: no ultrasonic, no steam, no harsh chemicals, and a warning to any jeweler before a repair involving a torch. The guide to cleaning jewelry safely lists methods by gem.
Ruby in history and folklore
The name comes from the Latin ruber, meaning red. Sanskrit texts called ruby ratnaraj, often translated as "king of precious stones." Burmese tradition held that a ruby inserted under the skin would make a warrior invulnerable, and some medieval Europeans believed a ruby darkened to warn its owner of danger. Ruby is the July birthstone and the traditional gift for a 40th wedding anniversary.
Frequently asked questions
Are ruby and sapphire the same mineral?
Yes. Both are corundum. Red corundum is ruby, and every other color is sold as sapphire, with the pink-to-red boundary set by trade custom.
Is a heated ruby fake?
No. A heated ruby is a natural ruby that has been treated with heat, a stable process that most rubies on the market have gone through. It should be disclosed, and unheated stones of similar quality cost more.
How can I tell if a ruby is glass filled?
Under magnification, glass-filled rubies often show flattened gas bubbles in the filler and a blue flash along fractures when the stone is tilted. A lab report gives a reliable answer.
What is the most prized ruby color?
A pure, intense red to slightly purplish red in a medium to medium-dark tone, often called pigeon's blood in the trade, commands the highest prices.