October 11, 2026

How to Tell If a Gemstone Is Real or Glass: Rubies, Sapphires, Emeralds

By Sasha Tarasova

Filed under Gemstone GuidesBuying GuidesCollecting & Value

Short answer: To tell if a ruby, sapphire or emerald is real, look at it under a 10x loupe for evidence of how it formed. Round gas bubbles and swirls suggest glass; curved growth lines suggest a flame-fusion synthetic; a flat join line around the girdle suggests a doublet. Natural stones show angular zoning, silk and crystal inclusions. Separating natural from lab-grown with certainty usually needs a gemmological laboratory.

"Is it real?" sounds like a yes-or-no question. With coloured stones it rarely is. A red stone in a ring can be natural ruby, heavily treated natural ruby, synthetic ruby, red spinel, garnet, a garnet-and-glass doublet, or plain red glass, and several of those were sold honestly in their day. This guide is about telling which one you have, as far as a loupe and some patience will take you, and knowing when to stop and send it to a lab.

One distinction first. Whether a gemstone is real is a different question from whether it is fine. A natural ruby can be dull, cloudy and cheap. For quality once you know the species, our guide to how to tell if a gemstone is fine covers colour, clarity and cut. This article is only about identity.

What does "real" actually mean for a gemstone?

There are five possibilities, and the words matter because they carry very different prices.

• Natural. Formed in the earth. It may still be treated, for example heated sapphire or oiled emerald.
• Synthetic (lab-grown, lab-created). Same chemistry and crystal structure as the natural stone, made in a factory. A synthetic ruby is ruby, but not a natural one. Flame-fusion synthetic ruby has been made commercially since the early twentieth century, so an old ring can easily contain one.
• Imitation (simulant). A different material that looks similar: glass, or a cheaper natural stone such as garnet sold as ruby.
• Composite. Two or more pieces joined together, usually a thin slice of real stone on a glass or cheaper base. Doublets have two parts, triplets three.
• Heavily treated natural. Natural material so altered that it sits in a category of its own, most notably lead-glass-filled ruby.

The FTC Guides for the Jewelry Industry (16 CFR Part 23) require that laboratory-created stones be described as such, with words like "synthetic", "laboratory-created" or "created", and not as "natural" or "genuine". In the secondhand market, though, a stone's paperwork is often long gone, and descriptions drift. Vintage "alexandrite" is a classic case, as our alexandrite guide explains.

What you need

A 10x jeweler's loupe, good daylight or a daylight-balanced lamp, a sheet of white paper, and patience. A penlight helps. That is the whole kit for the tests in this article. Everything else, refractometer, polariscope, spectroscope, dichroscope, is the gemmologist's toolbox, and we will say where it is needed.

How to spot glass

Glass is the oldest imitation of all, and in antique jewelry it is often honest: Georgian paste jewelry was made from high-lead glass deliberately, and good paste is collectable in its own right. The problem is glass sold as ruby, sapphire or emerald.

• Round gas bubbles. Spherical bubbles, alone or in strings, are the classic sign. Natural gems almost never contain perfectly round bubbles.
• Swirl marks. Curved, flowing lines where the glass was stirred or poured.
• Worn facet edges. Glass is soft, around 5 to 6 on the Mohs scale. After decades of wear its facet junctions round off and chip. A ruby or sapphire, at 9, keeps sharp edges much longer.
• Moulded look. Slightly concave facets, rounded edges straight from the mould, or a faint seam.
• Too much of a good thing. Large, flawless, vivid stones at small prices are rarely natural.

Glass also warms up faster than crystalline gems when held against the lip or in the hand. It is a weak clue, useful only for loose stones and only with practice.

How to spot a synthetic ruby or sapphire

Flame-fusion (Verneuil) synthetics, the cheapest and most common kind, grow as a boule from molten droplets. That process leaves two tell-tale features.

Curved striae

Look through the stone with the loupe, turning it slowly against a light background. Flame-fusion rubies and sapphires often show curved growth lines, fine arcs like the grooves on a record. Natural corundum grows in straight layers, so its colour zoning runs in straight lines and sharp angles, often following the hexagonal outline of the crystal. Curved lines in corundum are a strong sign of synthetic origin. They can be faint; tilting the stone and diffusing the light helps.

Gas bubbles

Flame-fusion synthetics may contain small gas bubbles, sometimes in clouds or rows. Natural corundum, instead, contains silk (fine needles of rutile crossing at angles), small crystals, and "fingerprints" of healed fractures.

Better synthetics are harder

Flux-grown and hydrothermal synthetics, made since the mid-twentieth century, lack curved lines and bubbles. Their inclusions, such as wispy flux "fingerprints" or metallic platelets, can resemble natural features to an untrained eye. If your loupe finds nothing suspicious, that is not proof of natural origin. It is simply the point where a laboratory takes over. Our antique sapphires guide covers the inclusions a natural Ceylon or Burmese stone typically shows.

How to spot a doublet or triplet

Composite stones are the great trap of antique coloured stones, because many were made in the late nineteenth and early twentieth centuries and set in solid gold alongside real diamonds.

The garnet-topped doublet and the red ring test

The most common is the garnet-and-glass doublet: a thin slice of red garnet fused onto a base of coloured glass. The garnet gives a hard, lustrous top surface; the glass supplies whatever colour was wanted, blue for "sapphire", green for "emerald", red for "ruby".

The red ring test works on loose stones or ones with an open back: place the stone table-down on white paper and look at it from the side. A garnet-topped doublet usually shows a red ring or halo around the edge, whatever its body colour, because you are seeing the garnet cap. A green or blue stone with a red rim is almost certainly a doublet.

Join lines and lustre differences

With the loupe, look at the girdle, the stone's outer edge, from the side. A composite often shows a flat plane or line where two pieces meet, sometimes with flattened bubbles trapped along it. Tilt the stone under a light: the crown may reflect with a different lustre from the pavilion, because they are different materials. In a closed setting the join can be hidden, which is one reason closed-back settings deserve extra scrutiny.

Emerald triplets and "soudé" stones

Emerald imitations include soudé emeralds ("soldered" in French): two pieces of colourless quartz or pale beryl joined by a layer of green cement or glass. Viewed from the side, the colour sits in a thin layer while the top and bottom look colourless. Immersing the loose stone in water can make the green layer obvious.

How to tell if an emerald is real

Real natural emeralds are almost always included. The trade calls the inclusions jardin, French for garden: wispy veils, fractures, and three-phase inclusions (tiny cavities holding liquid, a gas bubble and a crystal). They are also almost always treated with oil or resin to reduce the visibility of fractures. Our antique emeralds guide covers what is normal.

Signs to watch:

• Green glass shows round bubbles and swirls, and often a "too clean" look at a low price.
• Synthetic emerald, made commercially since around the mid-twentieth century, can look strikingly clean, or show wispy flux veils or chevron-shaped growth patterns (in hydrothermal material). These are difficult without experience.
• Doublets and soudé stones show join lines, colour confined to a layer, or the red ring.
• The Chelsea filter, a coloured filter that makes many natural emeralds glow pinkish-red, was once a popular check. Many synthetic emeralds react the same way, and some natural emeralds do not, so treat it as a sorting tool, not a verdict.

How to tell if a ruby is real

After everything above, rubies add two special cases.

Lead-glass-filled ruby

Low-grade, heavily fractured natural corundum is filled with high-lead glass so that the fractures almost disappear. The result looks like a clean, bright ruby at a very low price. Under the loupe, look for flashes of blue or orange within fractures as you tilt the stone, and gas bubbles trapped in the glass-filled areas. These stones are natural corundum in name only; they are fragile, can be damaged by jewelers' pickling solutions and even household chemicals, and have very little resale value. Our guide to which gemstone treatments are accepted and which kill value puts this at the bottom of the scale.

Ruby that is actually spinel or garnet

Red spinel and red garnet are real, natural gems that have been sold as ruby for centuries, sometimes innocently. Spinel in particular was not generally recognised as separate from ruby until around the late eighteenth century, and some of the world's most famous "rubies" are spinels. Our spinel guide explains how the two differ. Garnet is usually darker and more brownish or purplish; neither shows ruby's pleochroism (two different colours seen from different directions through a dichroscope).

Comparison: what you see, what it suggests

A step-by-step check at home

1. Clean the stone with warm water and mild soap, then dry it. Grime hides inclusions and doublet joins.
2. Look at facet edges with the loupe. Sharp and crisp on a century-old piece supports a hard stone; rounded and chipped suggests glass.
3. Look inside. Search for round bubbles, swirls, curved lines, silk, crystals and fingerprints. Rotate slowly and use a light background.
4. Look at the girdle from the side for a join line or change in lustre.
5. If loose or open-backed, do the red ring test on white paper.
6. Tilt a ruby under light to check for coloured flashes in fractures.
7. Note the setting. Hallmarks, gold quality and the maker's standard tell you what the piece should contain. A cheap stone in a fine setting is possible but deserves a question. Our guide to buying antique jewelry online covers reading listings and photographs for these clues.
8. Stop there. Do not scratch the stone, heat it, bite it or test it with acid. The loupe gives clues; a laboratory gives answers.

What needs a laboratory?

More than most people expect. A laboratory such as GIA uses spectroscopy, advanced microscopy and chemical analysis to answer questions a loupe cannot:

• Natural versus good-quality synthetic (flux-grown and hydrothermal).
• Whether a sapphire or ruby is heated, and whether other treatments such as diffusion or glass filling are present.
• The degree of clarity enhancement in an emerald.
• Geographic origin, where it can be determined at all.

For any stone worth more than a few hundred dollars, or one being bought for a meaningful price, a laboratory report is money well spent. For diamonds, which follow different rules and have different imitations, see our guide to how to tell if a diamond is real.

Ask the seller, including us

A good dealer should be able to say how each stone in a piece was identified: by laboratory report, by an in-house gemmologist with instruments, or simply assumed. The last answer is common and not always dishonest, but you deserve to hear it. Ask the question about any piece, whether that is our Edwardian old European cut diamond ring haloed in emeralds or our vintage diamond and ruby pinky ring, and expect a direct answer. Our piece on what a dealer should disclose sets out the full list.

Questions people actually ask

How can I tell if a ruby is real at home?

Use a 10x loupe. Round gas bubbles and swirls suggest glass; curved growth lines suggest a flame-fusion synthetic; blue or orange flashes in fractures suggest lead-glass filling; a red rim when the stone sits table-down on white paper suggests a garnet-topped doublet. Straight colour zoning and fine silk needles point towards natural ruby, but confirming natural origin and treatment usually needs a laboratory.

What is the difference between a fake emerald and a real one?

A real natural emerald almost always shows inclusions, called jardin, such as wispy veils and tiny three-phase inclusions, and is usually oil or resin treated. Glass imitations show round bubbles and swirls. Doublets and soudé stones show join lines or colour confined to one layer. Synthetic emeralds are real emerald in chemistry but lab-made; separating them reliably needs a laboratory.

Are synthetic rubies and sapphires fake?

Not in the chemical sense. A synthetic ruby or sapphire has the same composition and structure as the natural stone, so it is real corundum. It is not natural, and it must be described as synthetic, laboratory-created or created. Synthetic corundum has been made commercially since the early twentieth century, so antique and vintage jewelry can contain it. It is worth far less than natural corundum.

What is the red ring test?

The red ring test checks for a garnet-and-glass doublet. Place the loose or open-backed stone table-down on white paper and look from the side. If a red ring or halo appears around the edge, regardless of the stone's colour, you are likely seeing a garnet cap fused to a glass base. It is a quick, harmless screening test, not a full identification.

Can a jeweler tell if a gemstone is real?

A jeweler with gemmological training and instruments such as a refractometer, polariscope and microscope can identify most imitations, doublets and many synthetics. Separating natural from high-quality synthetic, and detecting treatments or origin, often needs a gemmological laboratory. Ask whether the person examining your stone is a trained gemmologist, and what instruments they used.

Does a real gemstone scratch glass?

Ruby, sapphire and many other gems are harder than glass and will scratch it, but so will many cheap imitations, including synthetic corundum and some other stones. The test proves little and can chip the stone you are testing. Do not scratch-test jewelry; use a loupe, and a laboratory when the answer matters.

If you want a natural ruby, sapphire or emerald with proper paperwork, our sourcing service will find one and tell you exactly how it was identified. More coloured-stone guides are collected under gemstones in the Journal.