Every gemstone page on this site ends with a "real or fake" note specific to that stone. This article collects the general tests behind those notes, the ones that work on almost any stone and need nothing more than what is in an ordinary house. None of them replaces a laboratory report for an expensive stone, but together they catch the great majority of what is sold as genuine and is not.
1. Temperature
Hold the stone to your lip or the inside of your wrist. Real crystal, quartz, corundum, beryl, garnet, feels distinctly cold and stays cold for a few seconds. Glass warms quickly; plastic and resin feel warm at once. This is the single fastest test and it eliminates most cheap fakes on its own. The exceptions are amber and pearl, which are naturally warm, and jet, which is a fossil wood.
2. Weight
Gems have characteristic densities. Sapphire, ruby, garnet and topaz are noticeably heavy for their size; glass is lighter than most of them; plastic is far lighter than any. If you can compare the suspect stone with a known real one of the same size, the difference in the hand is obvious. A yellow "sapphire" that weighs the same as a piece of citrine is citrine.
3. Bubbles and swirls
Look through the stone with a ten-power loupe under a lamp. Perfectly round bubbles, singly or in clusters, mean glass; so do swirl marks like those in a glass paperweight. Natural inclusions are never round: they are needles, feathers, crystals, fingerprints and clouds. A stone with no inclusions at all is either very expensive or synthetic.
4. Doubling
Many gems are doubly refractive, meaning light splits as it passes through them. Look through the table of the stone at the edges of the back facets with a loupe: in peridot, tourmaline, zircon and to a lesser degree sapphire, the edges appear doubled, like a slightly blurred line. Glass, garnet, spinel and diamond are singly refractive and show crisp single edges. This test separates peridot from green glass and topaz from citrine in seconds.
5. Colour change with angle
Pleochroic stones show different colours or tones from different directions. Turn the stone slowly under a lamp: tanzanite shifts from blue to violet, kunzite from pink to nearly colourless, tourmaline and iolite change tone, and sapphire and ruby change slightly. Glass and most synthetics look the same from every angle. A "tanzanite" that stays one colour is not tanzanite.
6. Colour that is too even
Natural colour is uneven. Amethyst and sapphire show zoning, bands of lighter and darker colour; aqeeq and turquoise vary across the piece; lapis has its pyrite flecks. A stone that is perfectly uniform in colour, especially a large and cheap one, is either glass, synthetic or dyed.
7. The acetone test for dye
Dip a cotton bud in nail-varnish remover and rub an unseen part of the stone, the back or the girdle. If colour comes off, the stone is dyed. This catches dyed howlite sold as feroza, dyed jasper sold as lapis lazuli, dyed quartz sold as ruby or emerald, and dyed agate of every colour. Real stone does not lose colour to acetone.
8. The saltwater test for amber
Dissolve two or three tablespoons of salt in a glass of water. Real amber floats; plastic, glass and copal sink. It is the standard test for amber and it works on beads still on their string, as long as the string is not weighing them down.
9. The hot-needle test for resin
Heat a needle and touch it to an unseen spot. Plastic and resin melt and smell chemical; amber smells of pine; real stone is unaffected. This is a last resort, because it marks the piece, and it should be done only on something you already suspect.
10. The magnet test for garnet
Most garnets are attracted to a strong neodymium magnet because of their iron content; glass, ruby and red spinel are not. Float the stone on a small piece of cork in water, or lay it on a smooth surface, and bring the magnet close. It is one of the few tests that separates garnet from its imitations without instruments.
When to insist on a report
Synthetic ruby, sapphire, emerald and spinel are chemically identical to the natural stones and pass every test above. For those four, and for any stone costing more than a modest sum, ask for a report from a recognised laboratory that states the stone is natural and lists any treatment. A seller who refuses, or who offers a certificate from a laboratory you cannot find, has told you what you need to know. For astrological use, a heated stone is usually accepted; a glass-filled, diffused or dyed one is not.