There is a particular thrill in opening an old jewellery box and lifting out a brooch or ring that has clearly lived a life. The gold may be worn thin at the claws, the setting slightly out of true, and the stones glow with a depth that modern mass-produced pieces rarely manage. But how do you tell whether that glow comes from a garnet, a piece of paste, or something considerably more valuable? The good news is that most vintage stones give themselves away through a handful of reliable clues — and you can spot many of them with nothing more than good light, a loupe, and a little patience.

Start With the Setting and the Era

Before you look at a single stone, look at how it is held. The setting is often the most honest witness to a jewel's age and intentions.

• Closed backs. Stones set in a fully closed collet — a cup of metal behind the stone — are typical of Georgian and early Victorian work. Foil was often placed behind the stone to brighten it, which means the colour you see may be partly the foil rather than the gem.
• Open backs. From the late nineteenth century onwards, jewellers increasingly left the back open so light could pass through, which flatters transparent stones and suggests a genuine gem was intended.
• Claw and grain settings. Fine, even claws and a well-finished gallery usually accompany better stones. Chunky, uneven claws with plenty of solder can be a sign of a later repair — or a cheaper original.
• Metal clues. Silver and pinchbeck (a copper-zinc alloy) were common in costume jewellery, while gold was generally reserved for pieces meant to last. Look for hallmarks, but remember they tell you about the metal, not the stones.

Visual Clues Under a Loupe

A ten-times loupe is the single most useful tool you can own. Hold the stone against a plain background and look through the table facet.

• Sharpness of facet edges. Genuine stones are cut and polished with crisp, straight junctions where facets meet. In glass and paste, these edges are often slightly rounded or wavy, and they wear down faster.
• Surface condition. Glass and paste scratch and abrade readily, so vintage imitations often show a network of tiny scuffs along facet edges and across the table. Diamonds and corundum are far more resistant.
• Bubbles. Round, perfectly spherical gas bubbles are a giveaway of glass. Natural inclusions are irregular — crystals, needles, feathers, clouds — and rarely perfectly round.
• Doubling. Look through the stone at the far facet edges. If you see a doubled image, you may be looking at a doubly refractive gem such as zircon, peridot, or moissanite.
• Colour zoning. Many natural stones show uneven colour in patches or bands, while glass tends to be uniformly tinted throughout.

Simple Tests at the Kitchen Table

None of these will give you a definitive identification on their own, but together they narrow the field quickly.

• The breath test. Breathe gently on the stone. Glass and paste fog up and clear slowly; diamond, and to a lesser extent most genuine gems, disperses the moisture almost instantly.
• The temperature test. Genuine stones feel noticeably cold to the touch at first, then warm slowly in the hand. Glass warms quickly and feels tepid from the start. This is unreliable in a warm room, so treat it as a hint rather than proof.
• The heft test. Compare similar-sized stones. A garnet feels unexpectedly heavy for its size, while glass feels light and slightly "dead" in the hand.
• The mirror test. For closed-back settings, place the stone face down on a mirror. If you can see the reflection through the stone, it is likely transparent and possibly genuine; a cloudy or blocked view suggests foil or an opaque backing.
• A torch, used carefully. Shine a narrow beam through the side of the stone. Glass often goes almost completely clear, while many gems retain their colour or reveal internal structure.

Stone by Stone: The Usual Suspects

Vintage jewellery leans on a fairly predictable cast of characters. Here is what to expect from each.

• Diamond. Look for adamantine lustre, sharp facet edges, and no bubbles. Under a loupe, natural diamonds usually show tiny internal crystals or feathers. Cubic zirconia and glass are the common stand-ins, but both lack diamond's crisp flash.
• Ruby and sapphire. Corundum is hard and takes a superb polish, so facet edges stay sharp even after decades. Expect fine silk-like inclusions or colour zoning. Paste rubies tend to look flat and overly pink or orange.
• Emerald. Almost always included — the jardin of internal fissures is a positive sign. Glass emeralds look suspiciously clean and often contain bubbles.
• Garnet. Deep, slightly brownish red with a warm glow. It is singly refractive, so shows no doubling, and it feels heavy for its size. Bohemian garnets were set in tightly packed clusters in Victorian work.
• Amethyst and citrine. Quartz shows characteristic internal breadcrumb-like inclusions and often a slight colour band. Glass versions are uniform and bubble-strewn.
• Opal. Genuine opal plays with colour as you tilt it; imitation opal sometimes has a painted or foil backing and a plastic-like feel.
• Pearl. Run a pearl gently across your teeth. Real nacre feels slightly gritty; glass and plastic feel smooth. Look for blemishes and overtones rather than perfect uniformity.

Knowing When to Stop Guessing

Home testing has limits, and the more valuable the piece, the more those limits matter. Two stones can look identical and behave similarly at the kitchen table: synthetic ruby and natural ruby, or a fine paste and a low-grade gem. If a piece is hallmarked gold, if the setting is finely made, or if the stones are large and clean, it is worth taking to a qualified gemmologist or a reputable independent jeweller. They can confirm identity with a refractometer, a spectroscope, or a microscope — tools that reveal what the eye cannot.

Above all, enjoy the detective work. Every vintage piece carries the fingerprints of its maker and its era, and learning to read those clues is half the pleasure of collecting. Handle everything gently, never scratch or file a stone to test it, and keep a loupe in your pocket. The more you look, the more your eye will learn.