James Whitfield
Why Synthetics Turn Up in Older Jewellery
It is tempting to assume that an old brooch, ring or pendant must contain natural stones. Age is not proof of origin. Flame-fusion synthetic ruby and sapphire were being made in commercial quantities by the early 1900s, synthetic spinel was widely used in the 1920s and 1930s, and flux-grown synthetic emerald arrived in the decades that followed. Paste, or glass, has been set into jewellery since the Georgian period and is still regularly mistaken for gemstone. Add a century of repairs, resets and replacements, and it is entirely possible to find a Victorian-style setting holding a twentieth-century synthetic. That is not bad news — it is simply information — but it helps to know what you are looking at.
Colour That Looks Too Even and Too Bright
Colour is often the first whisper. Natural corundum, emerald and tourmaline frequently show some variation in tone across the stone: faint zoning, a hint of pleochroism, a slightly duller area near the girdle. Synthetic material is grown in a controlled environment and can look remarkably flat — one even saturation from edge to edge, with a brightness that seems slightly suspicious for the age of the piece.
Look for the following when you turn the stone in daylight:
- No zoning: watch for bands, patches or streaks. Flame-fusion sapphire and ruby often show none at all.
- Too uniform a hue: natural blue sapphire usually shifts slightly in tone as you tilt it; synthetic blue can stay stubbornly identical.
- Colour swirls in glass: paste often shows curved streaks of colour and a watery, slightly cloudy look.
Inclusions, Bubbles and Curved Growth Lines
With a 10x loupe, clean the stone thoroughly and light it from the side as well as through the table. Internal features are your strongest clues.
- Gas bubbles: small, perfectly round bubbles, alone or in clouds, are a hallmark of flame-fusion material and of glass.
- Curved striations: sweeping, arc-like growth lines curve through flame-fusion corundum. Natural corundum tends to show straight or angular growth banding instead.
- Flux veils: wispy, drifting, veil-like feathers and fine residual flux are typical of flux-grown synthetic emerald and ruby.
- Missing natural character: if an emerald is claimed but there is no jardin at all, or a ruby shows none of the fine rutile silk you would expect in an untreated stone, be cautious.
- Stars that are too sharp: synthetic star sapphire and star ruby usually display a star that is almost too well defined and perfectly centred, while natural stars are softer and often sit slightly off-centre.
Cut, Polish and Surface Clues
Cutting tells its own story. Flame-fusion boules are small, so cutters often produce shallow, weight-saving stones with thin crowns and gently curved girdles. Glass is softer than most gemstones, so its facet edges round off and show abrasion, scratches and a slightly dull polish.
Then compare the stone with its setting. An old ring usually carries a century of wear: a thinned shank, worn claws, a soft hallmark. If the metal is tired but the facets are razor sharp with not a single nick, ask whether the stone was replaced at some point. The reverse is also useful: crisp, over-perfect cutting in a genuinely antique mount is a contradiction worth investigating.
Simple Checks You Can Make at Home
- Use a 10x loupe on a clean stone; fingerprints hide everything.
- Light from the side with a torch, not just from behind, so inclusions cast shadows.
- Weigh the stone in the hand: glass feels light and warms quickly, corundum stays cool for longer. This separates paste from gem material, though not natural from synthetic.
- Try a longwave UV lamp: synthetic spinel often glows a chalky blue-white, while natural spinel is usually inert. Treat this as a hint, never a verdict.
- Date the piece first — hallmark, style and setting — then judge the stone against that date.
When to Bring in a Gemmologist
The awkward truth is that synthetic corundum and natural corundum share the same hardness, refractive index and specific gravity. A loupe will catch the obvious cases, but it cannot settle every one. If a stone matters — for insurance, for resale, or simply because you love it and want to know — a microscope, refractometer, spectroscope and careful observation by a qualified gemmologist will give you a proper answer, often in writing. Enjoy the detective work, keep an open mind, and let the inclusions tell you their story.
