How to Judge Ruby Quality: Colour, Clarity, Cut and Treatments

What is a ruby, and what decides ruby quality?

A ruby is the red variety of corundum, a crystal of aluminium oxide (Al₂O₃). Pure corundum is colourless. A small amount of chromium replacing some of the aluminium turns it red, and the same mineral in every other colour is called sapphire. That shared family is why rubies sit alongside sapphires in our guide to coloured gemstones, and why ruby is also the traditional stone covered on our page about ruby as the July birthstone.

Ruby is 9 on the Mohs hardness scale, with a refractive index of about 1.762–1.770 and a specific gravity (density compared with water) of about 4.00. It is also dichroic, meaning it shows two slightly different colours depending on the direction you look through the crystal: one more purplish red, one more orangey red. That property matters when the stone is cut, as you will see below.

Unlike diamonds, rubies have no single worldwide grading system. Labs describe them rather than give them a letter grade, so judging ruby quality means weighing several things together:

• Colour: by far the biggest factor, often more than half of a ruby's value.
• Clarity: how visible the internal features are.
• Cut: how well the stone returns light and shows its colour face-up.
• Carat weight: fine rubies over 1 carat are rare, so price climbs steeply with size.
• Treatment: whether the stone is untreated, heated or filled with glass.
• Origin: certain sources carry a premium, though quality always comes first.

If you want to see what ruby looks like in real jewellery, our page on ruby gemstones in Melbourne shows the range, and every technical word here is also defined in our glossary of gem and jewellery terms.

How do you judge ruby colour?

Gemmologists break colour into three parts. Hue is the basic colour: red, with secondary hints of purple, pink or orange. Tone is how light or dark it is. Saturation is how strong or pure the colour is, from greyish or brownish to vivid. A ruby with the right hue but weak saturation looks dull; one with strong saturation but very dark tone looks almost black in low light.

Pigeon's blood is the trade name for the most prized ruby colour: a vivid, pure to slightly purplish red with strong red fluorescence, the glow that ultraviolet light in daylight produces in chromium-rich, iron-poor stones. Some laboratories, such as SSEF, Gübelin and GRS, add pigeon's blood as a colour term on reports when a stone meets their criteria, but the term is not used the same way by every lab, so treat it as a description rather than a measure of value.

Where does ruby end and pink sapphire begin? There is no fixed line. A stone with light tone and red hue is often called pink sapphire, and different labs and countries draw the boundary slightly differently. The same hue, tone and saturation framework applies to blue stones, which is why our guide to judging blue sapphire quality reads almost as a companion piece.

What clarity should a ruby have?

GIA classes ruby as a Type II coloured stone, meaning it usually grows with some inclusions (internal features). A completely clean natural ruby is rare, and buyers and dealers accept visible inclusions in ruby more readily than in diamond or aquamarine. The aim for jewellery is an eye-clean stone, with no inclusions you can see at normal viewing distance without magnification, or one whose inclusions do not spoil the colour or durability.

• Silk: fine needles of rutile (titanium dioxide). A little silk scatters light softly across the face and can actually improve the look. Heavy silk makes the stone hazy.
• Fingerprints and feathers: healed or open fractures. Healed fingerprints are common and stable; open feathers that reach the surface can be a weak point.
• Crystals: tiny mineral grains trapped inside. Fine if small and away from the surface.
• Colour zoning: bands of stronger and weaker colour, usually a cutting or colour issue rather than clarity.

Silk also tells a gemmologist something. Long, intact rutile needles usually suggest a ruby has not been heated to high temperature, because heat partly dissolves them. Broken, dotted silk is one clue that a stone has been heated, though a lab report is the only reliable confirmation.

Clarity matters most where the stone will take knocks. For a ring worn every day, avoid open feathers near the girdle, the thin outer edge, which is the part most exposed in everyday ruby engagement rings. A pendant or earrings can carry more visible inclusions without any practical risk.

Very dense, well-aligned silk is what creates the six-rayed star in a cabochon-cut star ruby. In that case, the inclusions are the point. Our page on star rubies and star sapphires explains how stars are judged differently from faceted stones.

How does cut affect ruby quality and size?

Most rubies are cut with a brilliant-style crown and a step-cut pavilion, called a mixed cut. Cutters often work to save weight, because ruby rough is precious, so cut quality varies widely. Ovals and cushions are the most common shapes, because they suit the way ruby crystals grow and keep the most weight from the rough. They are the same outlines described in our overview of gem and diamond shapes.

• Windowing: a pale, see-through patch in the centre where the pavilion is too shallow and light passes straight out the back. Common in lighter rubies.
• Extinction: dark areas that do not return light. A little gives contrast; large black patches, often from a deep pavilion, deaden the stone.
• Orientation: because ruby is dichroic, a cutter sets the table to show the purer red direction face-up. Poor orientation can leave a more orangey or purplish face.
• Depth: a heavy belly underneath adds carat weight but makes the stone look smaller for its weight.

Ruby is denser than diamond (about 4.00 against 3.52), so a 1 carat ruby looks noticeably smaller than a 1 carat diamond. Compare the millimetre measurements, not just the weight. Shape changes face-up size too, as our comparison of which shapes look largest for their weight shows for diamonds.

If you are weighing an oval ruby against an oval diamond as a centre stone, expect the diamond to look a little larger at the same carat weight.

Heated vs unheated ruby: which treatments matter?

Most rubies on the market have been treated in some way, and treatment affects value more than almost anything except colour. A good report from a lab such as GIA, SSEF, Gübelin, GRS or AGL will state whether a stone shows evidence of treatment. This is the same set of checks covered in the matching treatment checks for sapphire.

Lead-glass-filled rubies deserve special care. They begin as heavily fractured, low-grade corundum, and the glass can make up a large part of the stone. Many labs call the result a composite ruby. The glass can be etched by mild acids, even some household cleaners, and damaged by a jeweller's torch during repairs or resizing. Under magnification they often show flattened gas bubbles and a blue or orange flash along the filled cracks. They suit occasional-wear jewellery at a low price, but should be disclosed clearly and priced accordingly.

Heated rubies are the everyday standard and wear exactly like untreated ones. There is nothing wrong with heat; you simply should not pay an unheated price for a heated stone. Many birthday gifts, like the July birthday pieces set with ruby, use heated rubies for that reason.

Does ruby origin affect quality and price?

Origin shapes a ruby's chemistry, and chemistry shapes how it looks. Rubies formed in marble tend to be low in iron, so they fluoresce strongly and glow red. Rubies formed in basalt-related deposits are richer in iron, which darkens the tone and dampens fluorescence.

• Myanmar (Mogok): marble-hosted, historically the source of the most famous pigeon's blood stones. Commands a strong premium.
• Mozambique (Montepuez): a major source since about 2009, producing fine red stones, many low in iron.
• Thailand and Cambodia: iron-rich, often darker, slightly purplish to brownish red.
• Sri Lanka: often lighter, sometimes bordering on pink sapphire.
• Madagascar, Tanzania, Kenya, Vietnam, Afghanistan and Greenland: varied production with some fine material.

Australia is known worldwide for sapphire from the New England fields in New South Wales and the central Queensland gemfields, rather than ruby. Red corundum turns up there only occasionally, so almost all rubies sold in Australia are imported. If you are interested in local stones, our overview of natural coloured stones covers Australian sapphire and other options.

Only a well-equipped lab can determine origin, using inclusions, trace-element chemistry and spectroscopy. It is an expert opinion, not a certainty, and a fine ruby from a less famous source can look better than an average one from Myanmar. Judge the stone in front of you first.

Natural vs lab-created ruby: how can you tell?

Lab-created rubies (also called synthetic rubies) are real corundum with the same chemistry, hardness and refractive index as natural ruby, grown in a laboratory instead of mined. They have been made commercially since the early 1900s, using three main methods: flame fusion (the Verneuil process), flux growth and hydrothermal growth. Our guide to how lab-created rubies are grown explains each method.

• Flame fusion: curved growth lines and round gas bubbles under magnification; often very clean and inexpensive.
• Flux-grown: wispy, fingerprint-like flux inclusions and sometimes platinum flakes from the crucible; harder to separate from natural.
• Natural: straight, angular growth zoning, rutile silk and mineral crystals.

A flawless-looking ruby at a low price is often lab-created, but some lab-created stones are hard to identify without lab testing. Lab-created rubies cost a small fraction of natural rubies and are a sound choice if you want the colour and durability without the cost; they should simply be sold as lab-created. The same guide also compares lab-created sapphire and emerald.

Is ruby durable enough for an engagement ring?

Yes. Hardness (resistance to scratching) and toughness (resistance to chipping and breaking) are different things, and ruby is strong on both. It has no cleavage, the tendency to split along flat planes that diamond has, so it resists knocks very well.

Ruby is the hardest natural gem after diamond, which is why it appears so often in ruby engagement ring designs. For a direct comparison of sparkle, cost and wear, see ruby versus diamond for everyday wear and our guide to choosing between a ruby or sapphire for an engagement ring. The one exception is lead-glass-filled ruby, which is not suited to daily wear or to repairs involving heat.

Ruby price per carat in Australia: what affects it?

Ruby prices climb steeply with colour, size and lack of treatment. A fine 2 carat ruby can cost many times more per carat than a fine 1 carat. These are broad approximate ranges only; they move with the market and vary greatly from stone to stone.

Origin, a report naming pigeon's blood and an unheated finding can each add a further premium at the top end. Our page on our ruby gemstone range gives a feel for what different qualities look like side by side.

A fine natural ruby can cost as much per carat as a good diamond, while commercial rubies cost far less. If you are weighing the two for a centre stone, our page on how ruby and diamond prices compare sets out the trade-offs in size, colour and budget.

How to judge ruby quality: step by step

1. Read the lab report. Confirm it says natural or lab-created ruby, and check the treatment line: no heat, heated, or glass-filled.
2. Judge colour in several lights. Look for a pure to slightly purplish red at medium to medium-dark tone, with no brown or grey.
3. Check the face-up appearance. Tilt the stone and look for a pale window in the centre or large dark areas.
4. Check clarity by eye, then with a loupe. Aim for eye-clean; note any open fractures near the surface or girdle.
5. Compare measurements, not just carats. A deep stone can weigh more but look smaller.
6. Match price to treatment and size. A heated stone should not carry an unheated price, and a glass-filled stone should be priced far lower.
7. Plan the setting. Choose claws or a bezel that protect the girdle.

If a report uses a term you have not met, such as flux healing, composite or no indications of heating, look it up in our plain-English jewellery dictionary before you compare stones.