The only certain way to know whether a diamond is real is to have it tested by a jeweler with proper instruments or by a gem lab. Home observations such as the fog test or reading print through the stone can raise doubts about obvious fakes, but none of them can confirm a diamond. Thermal and electrical testers separate diamond from most simulants, and only laboratory equipment can tell a natural diamond from a lab-grown one.
Three questions, not one
"Is it real?" can mean three different things.
- Is it diamond at all, or a simulant such as cubic zirconia, moissanite, white sapphire, white topaz or glass?
- If it is diamond, is it natural or lab-grown?
- Has it been treated, such as with fracture filling or HPHT color processing?
Home observations can only hint at the first question. Jewelers' testers answer the first question for most stones. The second and third questions need a gem lab.
Home observations and their limits
These checks are popular because they need no equipment. Each works on some simulants and fails on others.
The fog test
Breathe on the stone to fog it. Diamond conducts heat very well and disperses the fog almost at once, while glass and CZ stay fogged a little longer. The test is unreliable: a dirty diamond with a film of oil holds fog, moissanite clears just as fast as diamond, and timing differences are small and hard to judge.
Reading print through the stone
Place a loose round stone table-down on a page of printed text. A well-cut round brilliant bends light so strongly that the letters blur into an unreadable smear. Glass, quartz and some CZ may let you see the letters. The test only works on loose, well-cut rounds. Fancy shapes, shallow diamonds and moissanite can give misleading results, and it cannot be done on a mounted stone.
The dot test
A variation of the print test uses a dot drawn on white paper. Through a diamond, the dot usually disappears; through a simulant, it may show as a circle. The same limits apply.
Looking through a loupe
A 10x loupe shows useful clues to someone who knows what to look for.
- Most natural diamonds have small inclusions. A stone that is perfectly clean and very large deserves a closer look, though clean diamonds do exist.
- Diamond facet edges are sharp and crisp. Rounded, worn edges on an older ring suggest a softer simulant.
- Doubled facet junctions seen at an angle through the crown point to moissanite or another doubly refractive gem. Diamond is singly refractive.
- Gas bubbles and swirl marks point to glass.
- A frosted or finely faceted girdle is typical of diamond, and a laser inscription with a report number can be checked online.
Other home tricks to avoid
The water test (diamonds sink) only shows that a stone is denser than water, which almost every gem is. Scratching glass with the stone proves little, since many simulants scratch glass too, and trying to scratch a stone or hit it can chip a real diamond along its cleavage. Heating a stone and dropping it in cold water risks cracking any gem. These tests are not worth the risk.
Thermal testers
Diamond conducts heat better than any other natural gem. Handheld thermal testers, common since the late 1970s, use a heated metal tip. When the tip touches the stone, the tester measures how quickly heat flows away. Fast heat flow reads as diamond.
Thermal testers reliably separate diamond from CZ, glass, sapphire, topaz, spinel and most other simulants. Their weakness is moissanite, which also conducts heat well and reads as diamond on a thermal-only tester.
Accurate use takes some care. The stone and tester should be at room temperature, the tip must touch a facet squarely, and very small stones can give false readings. Touching the metal setting by mistake also gives a false positive.
Electrical conductivity testers
Most gem moissanite conducts electricity, and nearly all diamonds do not. Testers designed after moissanite arrived in the late 1990s measure electrical conductivity, often combined with a thermal probe. A stone that passes the thermal test and also conducts electricity is flagged as moissanite.
Natural blue Type IIb diamonds, which contain boron, and some boron-bearing lab-grown diamonds also conduct electricity. A combined tester may misread these real diamonds as moissanite, so any blue or gray stone that triggers the moissanite alert needs further testing. The diamond chemistry and formation page explains Type IIb diamonds.
Other tests a jeweler can run
Beyond handheld testers, a gemologist has several tools.
- Specific gravity. A loose stone can be weighed in air and water to find its density. Diamond is about 3.52, CZ 5.6 to 6.0 and moissanite about 3.21.
- Refractometer. Diamond's refractive index of 2.42 is above the range of a standard refractometer, so a stone that gives a reading cannot be diamond.
- Ultraviolet light. Many diamonds fluoresce, often blue, under longwave UV. Some simulants react differently. Fluorescence varies widely, so it is a supporting clue, not a test.
- Microscope. Higher magnification shows inclusions, doubling and surface features more clearly than a loupe.
The gemological instruments page describes these tools.
Natural versus lab-grown
Lab-grown diamonds are diamond. They pass every thermal tester and every standard electrical test (except boron-bearing stones), and they look the same as natural diamonds under a loupe. Separating them takes laboratory methods.
- Screening devices built for the trade check stones by spectroscopy and refer suspect ones for full testing.
- Full lab testing uses infrared and photoluminescence spectroscopy, deep ultraviolet fluorescence imaging that reveals growth patterns, and microscopy for growth-related inclusions.
Many lab-grown diamonds carry a laser inscription on the girdle stating that origin, and many are sold with reports that say so. If a stone has no report, a lab can issue one. The diamond grading labs page lists the main labs.
Treatments
Some treatments are visible with a microscope. Fracture filling often shows a "flash effect," colored flashes along filled cracks when the stone is tilted, and laser drill holes appear as fine tunnels from the surface to an inclusion. Color treatments such as HPHT processing and irradiation usually need spectroscopy. The diamond treatments page covers each method.
Why professional testing matters
Home observations catch obvious glass and some CZ, but they cannot rule out moissanite, cannot detect lab-grown diamonds and cannot reveal treatments. A misjudged stone can mean overpaying for an imitation or undervaluing a real diamond. For any stone that matters to you, ask a jeweler with a combined tester for a quick check, and send larger or costlier stones to a gem lab for a report. The diamond certificates guide explains what that report will show.
Frequently asked questions
Does the fog test work on diamonds?
It can hint that a stone is glass or CZ, but it is unreliable. Dirty diamonds hold fog and moissanite clears as fast as diamond.
Can a diamond tester detect lab-grown diamonds?
No. Standard thermal and electrical testers read lab-grown diamonds as diamond. Separating them needs laboratory equipment.
Can moissanite fool a diamond tester?
It can fool a thermal-only tester. A tester that also measures electrical conductivity will usually identify it.
What is the most reliable way to test a diamond?
A gem lab report. Labs confirm whether a stone is diamond, whether it is natural or lab-grown and whether it has been treated.