How to use a diamond tester

A diamond tester is easy to use badly. The device is rarely the problem. The stone is warm from a finger, there is skin oil on the table, the tip slips onto a prong, or the battery is half dead, and the lights say something the stone is not. This guide is the routine that removes those errors, and how to read what you see afterward. If you have not bought a tester yet, start with which tester to buy. For the physics behind the lights, see how diamond testers work.

Before you touch the stone

1. Know which tester you have

Look at the box or the manual for two words: moissanite and metal. A thermal only pen has neither mode and reads moissanite as diamond. A multi tester has a moissanite indicator, and good ones have a metal alert. Everything below assumes you know which of the two is in your hand, because the same green light means different things on each.

2. Fresh battery

Many cheap pens give weak or drifting readings on a tired battery before they stop working completely. If the tester has a low battery light, believe it. If it does not, start with new batteries anyway. It is the cheapest way to remove a whole class of wrong answers.

3. Warm it up and set it

Thermal testers need the tip at operating temperature. Most show a ready light after a short warm up. Some older pens ask you to set a dial for room temperature or stone size. Follow the manual for your model; there is no universal setting.

4. Clean, dry, room temperature

Wash the stone with warm water and a drop of dish soap, rinse, and dry it with a lint free cloth. Then wait a few minutes. A stone straight off a warm hand, or still damp, changes how heat flows out of the tip. Our guide to cleaning a diamond ring has the gentle method.

The test itself

1. Hold the stone steady. A loose stone goes table up in a stone holder or on a clean, hard surface. A ring goes in your hand or a ring clamp with the stone facing up.
2. Complete the circuit if the manual says so. Many units need your thumb on a metal plate on the pen body, and for mounted stones some ask you to hold the metal of the ring with the other hand. Skipping this is a common cause of no reading at all.
3. Aim at the table facet, at a right angle. The tip should land flat on the largest facet on top, perpendicular to it, not tilted onto a facet edge.
4. Use light, steady pressure. Press until the retractable tip moves in, then hold still. Pushing harder does not help and can slip the tip onto metal.
5. Do not touch the prongs or the band. On a mounted stone this takes practice. If your tester has a metal alert, a metal warning means you missed; reposition and try again.
6. Read once, lift, wait a few seconds, repeat. Two or three consistent readings are a result. One reading is not.

How to read the result

Every brand uses its own lights and sounds, so check your manual for the exact colors. The logic is the same across them.

Notice what is missing from the right hand column: there is no reading for lab grown. Lab grown diamond is diamond and tests the same. The reasons are in do lab grown diamonds pass a diamond tester.

False results and what causes them

The boron case is real but uncommon: some natural blue diamonds and some lab grown stones contain boron, conduct a little current, and can trip the moissanite channel. If a stone you believe is diamond keeps reading moissanite, that is a reason to see a lab, not a reason to trust the beep.

Loose stones vs set stones

Loose stones

Easiest case. Clean, place table up in a holder, test the table. Very small stones are the exception: when the facet is smaller than the probe tip, contact is unstable and results scatter. Testers usually state a minimum size; trust it.

Set stones

• Solitaires are fine if you can reach the table without touching prongs. Clamp the ring so your hand is not shaking.
• Halo and pavé stones sit so close to metal that contact is hard. Test the center stone and treat side stones as unknown unless the tester is rated for melee.
• Bezel settings leave only the table exposed, which is fine, as long as the tip stays inside the metal rim.
• Doublets (a thin top glued to a different base) can test as the top material only. A loupe check of the girdle is the way to catch them.

Back the beep up with a second check

A tester answers one question. Pair it with something that answers a different one. A 10x loupe shows doubled back facet edges in moissanite, which diamond does not have. A gem scale catches cubic zirconia, which is far heavier than diamond for its size. Our guide to telling if a diamond is real lists the home checks, and signs of a fake diamond covers settings and stamps. Skip the scratch test; a real diamond can still chip, as explained in diamond hardness.

Our take

The routine is boring on purpose: fresh battery, clean dry stone at room temperature, tip flat on the table, no metal, three readings. Done that way, a multi tester is a reliable sorter for diamond, moissanite and everything else. It is still a sorter. For origin and grade, take the stone somewhere that can give you a report; see where to get a diamond tested.

Worth being clear: everything here, and the app itself, is an educational reference. At-home tests narrow things down, they do not certify a stone. Before you buy, sell or insure anything of real value, get it looked at by a certified gemologist or an independent appraiser.