Testing a Stone at Home: Hardness, Magnet, Heat and Loupe

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What these tests can (and cannot) do

You have a stone and would like to know whether it matches what the label says. A few simple actions give clues. None is enough on its own, and none replaces the opinion of a gemmologist or a laboratory. These tests help spot inconsistencies, for example a “stone” that is actually glass. For safety, we do not describe any acid or flame test: they damage stones and are dangerous.

Golden rule: always test on an inconspicuous area (the underside, the edge), never on the visible face, and never on a precious stone or a mounted piece of jewellery without a professional’s advice.

Test 1: looking through a loupe

1. Take a loupe (10x magnification if possible) and a white light.
2. Look inside the stone, then at the surface, from different angles.
3. Look for round bubbles: in glass, they are often perfectly round or elongated and clearly visible. In natural stones, cavities are generally irregular.
4. Observe the colour: a perfectly uniform, vivid colour may indicate dye or treatment. Natural stones often vary from one area to another.
5. Check the fissures and drill holes: colour concentrated in a crack sometimes indicates dye.

A single bubble is not conclusive: some natural stones contain gas inclusions. It is the combination of clues that matters.

Test 2: warmth to the touch

Crystalline stones conduct heat well and feel cool to the touch for longer than glass or plastic, which warm up more quickly.

1. Leave the stone for a while in the room, at room temperature.
2. Press it against your cheek or earlobe, which are more sensitive than your fingers. Never put it in your mouth: some minerals are toxic.
3. Note the sensation of coolness and how long it lasts.
4. Compare with a piece of glass and a plastic object of similar size.

Limits: this test does not work for opal, amber, jet, coral and pearls, which are poor heat conductors. It depends on the size of the stone and the room temperature. A “cool” result does not prove authenticity.

Test 3: hardness (with care)

The Mohs scale ranks minerals from 1 to 10 according to their ability to scratch one another. Everyday objects serve as reference points, with values that vary slightly between sources:

Object	Approximate hardness
Fingernail	about 2 to 2.5
Copper coin	about 3
Window glass	about 5.5
Steel blade	about 5 to 5.5
Steel file	about 6.5

1. Reverse the test: instead of scratching the stone, try to scratch a clean piece of glass with an inconspicuous edge of the stone. If the glass scratches, the stone is harder than 5.5, as is the case with quartz (7).
2. For a soft stone, try your fingernail on a hidden area. Selenite (hardness 2) scratches with a fingernail.
3. Never try to scratch the visible face or a fine stone. A scratch is permanent.

Take care with cuts: handle sharp objects carefully, away from children. Keep in mind that a stone known as a “glass stone” (such as goldstone or redstone, which are glass) has a hardness of around 5 to 6: this test cannot distinguish a soft stone from glass.

Test 4: the magnet

Bring a magnet close to the stone. Most stones do not react. A few special cases:

• Magnetite is strongly attracted.
• Natural hematite is weakly magnetic, sometimes not at all. “Hematine” beads, imitations made from compressed powder, may on the contrary stick firmly to the magnet.
• Most coloured stones (quartz, agate, jasper…) do not react.

The magnet test is therefore only a simple clue. A strong attraction to a bead sold as hematite should arouse curiosity, but the test proves nothing on its own.

Safety: use an ordinary magnet. Neodymium magnets are powerful, can pinch fingers and can shatter. Keep magnets away from pacemakers, bank cards and electronic devices.

Test 5: dye

To check whether a stone is dyed (dyed howlite, dyed agate), dampen a cotton bud or a white cloth with a little water, then gently rub an inconspicuous area. If colour comes off, the stone is dyed. Dry it immediately. Do not carry out this test on a water-sensitive stone, such as natural turquoise or selenite. The absence of traces does not rule out all dye: well-fixed dyes do not come off.

Common mistakes and exercise

• Drawing a conclusion from a single test.
• Scratching the visible face of a stone.
• Using acid, vinegar or a flame: dangerous and destructive.
• Forgetting the light: a stone should be judged in daylight.

Exercise: gather three objects (a quartz pebble or a piece of rock crystal, a piece of glass, a plastic object) and put them through the heat and hardness tests. Note what you observe. You will then have a basis of comparison for your future stones. To go further, read our guide to telling real stones from fake ones.

Frequently asked questions

Also read

The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Step-by-step tutorials.