To tell if a ruby is real, look inside it with a 10x loupe under a bright lamp and then look at its fractures. A natural ruby almost always carries something: fine needles of rutile called silk crossing at 60 degree angles, angular color zoning, small crystals, or wispy healed fractures that look like fingerprints. A flame-fusion lab ruby, which is what sits in most class rings, birthstone rings and mid-century cocktail rings, shows curved growth bands like the grooves on a record and sometimes tiny round gas bubbles. A glass-filled ruby, by far the most common "ruby" sold since about 2004, shows a blue or orange flash inside its fractures as you tilt it and flattened bubbles trapped in the filler. Glass imitations show round bubbles, swirls and worn facet edges. Most red stones in inherited and bargain jewelry are lab-grown, glass-filled or glass, and that is simply how the trade has worked for a century. A natural ruby of good color is rare, and a clean one carat natural stone can be worth from several hundred to many thousands of dollars.
Quick answer
- Curved growth lines or round bubbles mean flame-fusion synthetic. Straight angular zoning, silk and fingerprints mean natural.
- Blue or orange flashes inside cracks and a stone that looks too clean for its low price mean lead glass filling.
- Lab ruby is worth about $2-30 per carat loose. Glass-filled composite is about $1-10 per carat. Natural heated ruby of commercial quality is about $200-2,000 per carat; fine unheated Burmese stones are $5,000-30,000 per carat and up as of late 2026.
How to check yours, step by step
- Clean it and set up the light. Warm soapy water, a soft toothbrush, dry it. Use one bright lamp against a dark background, and if you can, a glass of water to immerse the stone, which cuts surface reflections and makes internal features easier to see.
- Look at the facet edges. Ruby is corundum, hardness 9. After decades in a ring, its facet junctions are still crisp and the surfaces flat. Glass is soft, so its edges look rounded under the loupe, with tiny chips and scratches from ordinary dust and sand. Garnet, at hardness 7 to 7.5, sits in between and usually shows some wear.
- Look for curved lines. Tilt the stone and look through the table and the pavilion. Flame-fusion synthetic ruby, made by the Verneuil process since the early 1900s, grows in a boule that leaves faint curved color bands. They can be very subtle in red stones, which is where the glass of water helps. Natural ruby shows color zoning in straight bands with sharp corners.
- Look for bubbles. Round, isolated gas bubbles mean glass or flame-fusion synthetic. Natural ruby has no gas bubbles. What natural ruby has instead is silk: sets of fine needles in straight lines, often three directions crossing at 60 degrees, plus small included crystals and fingerprints.
- Check the fractures for flash. Glass-filled ruby is low-grade corundum, heavily cracked, that has been soaked in lead glass so it looks transparent. Under the loupe, rock the stone and watch the cracks: a blue flash, sometimes orange, appears along the fracture planes. Look also for flattened gas bubbles sitting inside the cracks, and for a slightly cloudy or cloudy-bright look. A large, bright, affordable ruby is almost always this material. It is real corundum, but worth a tiny fraction of untreated ruby, and a jeweler's torch or even a dishwasher can damage the filler.
- Use a UV light if you have one. Under long-wave UV, ruby fluoresces red because of its chromium. Both natural and synthetic do, and synthetics often glow more strongly because they lack the iron that dampens the glow in natural Thai and African stones. Garnet does not fluoresce. Glass usually does not. So a red glow says corundum, not natural; no glow says look harder at garnet, glass or spinel.
- Consider the look-alikes. Red garnet is darker and browner, with a glassy luster and no fluorescence. Red spinel is a real gem in its own right, often cleaner and slightly pinker, and valuable; a strong red spinel can be worth hundreds per carat. Rubellite tourmaline is pinker and shows strong doubling of the back facets through the loupe. Victorian "garnet-topped doublets" have a thin garnet crown glued to red glass; look for a seam near the girdle and bubbles in the glue layer.
- Read the setting. A 10K, 14K, 18K or platinum stamp tells you the setting was made to a jewelry standard, not that the stone is natural. Plenty of gold rings hold lab ruby. Our guide to how to read jewelry hallmarks covers the marks and how to tell if gold is real covers the metal.
What ruby is, and what gets sold as ruby
Ruby is red corundum, aluminum oxide colored by chromium. Pink corundum is called pink sapphire; the boundary is argued over and it matters for price. Hardness 9 is what makes the surface test useful. The chromium is also what makes the UV test useful and what gives a fine ruby its glow in daylight.
| Stone | What it is | What the loupe shows | Typical value, as of late 2026 |
|---|---|---|---|
| Natural ruby, unheated | Mined corundum, no treatment | Silk, angular zoning, crystals, fingerprints | About $2,000-30,000 per carat for good red, far more for fine Burma over 2 carats |
| Natural ruby, heated | Mined corundum heated to improve color and clarity, the trade norm | Same features, silk partly dissolved into dots and streaks | About $200-5,000 per carat |
| Glass-filled ruby | Heavily fractured corundum filled with lead glass | Blue or orange flash in cracks, flattened bubbles, cloudy look | About $1-10 per carat |
| Lab-grown, flame fusion | Synthetic corundum, mass produced since about 1905 | Curved growth bands, occasional round bubbles | About $2-15 per carat |
| Lab-grown, flux or hydrothermal | Chatham, Kashan, Ramaura and similar, grown slowly | Dense bright veils, flux residue like breadcrumbs, sometimes chevron growth | About $30-150 per carat |
| Red spinel, natural | A different mineral, historically mistaken for ruby | Clean, small octahedral crystals, no silk, no doubling | About $100-1,500 per carat for good red |
| Garnet | Pyrope or almandine, dark red to brownish | Needles and crystals but no fluorescence, often worn edges | About $5-50 per carat |
| Glass | Red glass, "paste" in older pieces | Round bubbles, swirls, rounded facet edges | A few dollars per stone |
| Garnet-topped doublet | Thin garnet crown on red glass | Seam near the girdle, bubbles in the glue plane, red glass visible from the side | About $5-30 per stone |
Color decides the price of a natural ruby more than anything else. The target is a pure, saturated red with a slight blue undertone, the look the trade calls pigeon blood. Stones that lean orange, brown or purple sell for a fraction. Buyers accept silk and fingerprints in a ruby, but cracks that reach the surface cut the value sharply. Heating is accepted and most ruby sold is heated; an unheated stone with a lab report from GIA, AGL, SSEF or Gübelin commands roughly two to five times the price of a heated stone of the same look.
Natural, lab-grown, filled: a side by side
| Clue | Natural ruby | Lab-grown ruby | Glass-filled ruby | Glass or garnet |
|---|---|---|---|---|
| Facet edges after years of wear | Crisp | Crisp | Crisp but cracks reach the surface | Rounded, chipped |
| Inclusions | Silk, crystals, angular zoning, fingerprints | Curved bands and round bubbles, or dense veils | Many cracks, flash in cracks, flattened bubbles | Round bubbles and swirls, or needles with no glow |
| Long-wave UV | Red, weak to strong | Red, usually strong | Red, patchy | Inert |
| Looks too clean for the price | Rare over a carat | Common | No, usually cloudy or cracked | Common in glass |
| Price for a 2 carat stone | Hundreds to tens of thousands | Under $50 | Under $30 | Under $20 |
| Typical jewelry | Fine gold and platinum, old and new | Class rings, mothers rings, 1920s-1980s gold | Modern silver and low-karat gold, online bargains | Costume, Victorian paste, Bohemian garnet |
Separating a clean natural ruby from a flux-grown synthetic, and a heated stone from an unheated one, is beyond a loupe and beyond a photograph. Those two questions decide whether a stone is worth $100 or $10,000, which is why a gem lab is part of the process for any natural-looking stone over about a carat. The same logic applies to sapphire, which is the same mineral in other colors, and to emerald, where the filling question is even more important.
Star rubies and cabochons
A star ruby shows a six-rayed star from silk reflecting a single light source. Linde in the United States sold millions of synthetic star rubies from the 1940s to the 1970s, and they are easy to spot: a perfect, sharp star sitting right on the surface of a very even, opaque, slightly purplish red cabochon, often with a flat polished back and a tiny L. A natural star ruby has a softer star that seems to float, a body that is usually grayish, pinkish or uneven, and a back that is often rough. Natural star rubies of decent color are worth about $100-2,000 per carat; Linde stars sell for about $20-100 per piece, more in a good period gold setting.
How WorthPic reads ruby photos
WorthPic runs on the newest and most capable AI model available, tuned for exactly this kind of identification. When you upload a ruby, it reads the things a gemologist looks at first: how crisp the facet junctions are, whether there are bubbles or curved bands, whether the cracks show a flash, the depth and hue of the red under your lighting, the quality of the cut, and the marks and style of the setting. From those it names the most likely category, natural, lab-grown, glass-filled or imitation, gives an honest value range for that category and explains which detail carried the most weight. Where a feature is not visible, for instance when the stone was photographed dry and the interior is all reflections, it says so and asks for the specific shot that would settle it, usually a macro through the table with the stone immersed in water. It identifies and estimates. It does not grade stones, does not issue reports and does not vouch for a stone. It cannot see heat treatment, and it cannot separate a clean natural ruby from a flux-grown synthetic. For any stone that looks natural and is over about a carat, a lab such as GIA, AGL, SSEF or Gübelin is what settles the question and the price.
Common mistakes
- Trusting the scratch test. Synthetic ruby scratches glass just as well as natural ruby, and the test can chip the stone.
- Assuming an 18K setting means a natural stone. Mid-century gold rings with synthetic ruby are everywhere.
- Taking a strong red UV glow as proof of natural. Synthetics usually glow more strongly, not less.
- Reading a large, bright, clean ruby at a low price as a bargain. It is almost always glass-filled.
- Letting a jeweler retip or resize a ring with a filled ruby without telling them. The torch can turn the stone cloudy or crack it.
What to photograph
- The stone face-up in daylight near a window, and again under one lamp, so the hue and fluorescence-driven glow show.
- A macro through the table with the stone in a shallow glass of water, which brings out curved bands, zoning and the flash in filled cracks.
- A close, angled shot of several facet junctions to show whether edges are crisp or rounded.
- A side view at the girdle, which catches doublet seams, surface-reaching cracks and color concentrated at the edges.
- The purity mark or maker's mark inside the ring or on the clasp.
- For a cabochon, the star under one point of light and the back of the stone.
When to get it looked at in person
- A gem lab such as GIA, AGL, SSEF or Gübelin issues a lab report stating natural or synthetic, treatment and sometimes origin. Expect about $100-300 for a standard report, more with an origin opinion. Worth it for any natural-looking ruby over about a carat and essential before selling a stone described as unheated, Burmese or pigeon blood.
- An independent gemologist with a microscope and refractometer can separate natural, synthetic, filled and imitation in minutes for about $30-100, a sensible first step for a box of inherited rings.
- A jeweler with a loupe will usually spot glass, Linde stars, obvious flame-fusion bands and glass filling, and can test the metal.
- An auction house jewelry department is the route for a natural stone over two carats in a signed or period setting, once a lab has pointed the same way.
For the rest of the box, how to tell if a diamond is real covers the white stones that often sit next to a ruby in the same ring.