Do black diamonds pass diamond tester?

Black diamonds, also known as carbonado diamonds, are a type of natural diamond that has a black or dark gray color. They are one of the rarest and most unusual types of diamonds. Black diamonds get their dark color and opacity from a high amount of inclusions, which are tiny imperfections within the diamond’s crystal structure. This leads to some key questions – can black diamonds pass a diamond tester? And what causes black diamonds to have different properties than colorless or more common diamonds?

What is a diamond tester and how does it work?

A diamond tester is a small electronic device used to distinguish real diamonds from diamond simulants like cubic zirconia. Diamond testers work by using thermal conductivity – a diamond’s ability to conduct heat. Diamonds are very efficient conductors of heat, while simulants are not.

To test a diamond, the diamond is placed on the tester’s probe tip. The tester sends a small electrical current through the tip, heating it up. This heat is conducted rapidly through the diamond if it is real. This heat conduction then registers on the tester’s sensors and lights up the “diamond” indicator. Simulants will not conduct heat as well, so they will not light up the tester.

Thermal conductivity

Thermal conductivity is a property that describes how well a material transfers heat. Substances like metals are good thermal conductors, while insulators like plastic do not conduct heat well. Diamonds have very high thermal conductivity, rated between 900–2,320 watts per meter kelvin. This means they can transfer heat energy quickly without becoming hot themselves. This physical property is what allows diamond testers to identify real diamonds.

Electrical resistance

A diamond’s electrical resistance also plays a role in diamond testers. Diamonds have very low electrical resistance, while simulants are electrical insulators. When the tester’s probe tip heats up and transfers that heat into the diamond, the diamond’s conductive nature creates a small electrical current. This current completes the circuit in the tester, confirming the stone is a true diamond.

Do black diamonds conduct heat like normal diamonds?

Yes, black diamonds have the same exceptional thermal conductivity and electrical properties as colorless or fancy colored diamonds. This means they will test as genuine diamonds on thermal conductivity and electrical resistance diamond testers.

A black diamond’s dark color and opacity comes from microscopic black inclusions distributed throughout its structure. But these black flecks are still carbon, the same material that makes up a diamond crystal lattice. So they do not impede diamond’s ability to rapidly conduct heat or electricity.

High thermal conductivity

Natural black diamonds inherited the same strong capacity for thermal conductivity as other diamonds, allowing them to quickly channel heat through the crystal lattice. So when placed on a thermal probe, black diamonds will heat up and pass the conductivity test just like clear diamonds or diamonds with more typical colors like yellow or pink.

Low electrical resistance

Additionally, the dark inclusions in black diamonds do not raise electrical resistance enough to affect diamond tester results. Black diamonds remain excellent electrical conductors compared to simulants. When electrical current is applied by a diamond tester, black diamonds will complete the circuit and register as genuine, just like a colorless diamond would.

What makes black diamonds black?

Black diamonds get their unique dark gray or black color from black or dark green crystal inclusions distributed throughout the diamond’s structure. Under magnification, these black flecks can be seen interspersed between the carbon crystal lattice.

These dark inclusions are also made of carbon and are believed to come from primitive carbon crystals that formed under intense pressure. Over millions of years underground, these black carbon flecks were incorporated into the diamond’s crystal growth structure. This resulted in the opaque, dark appearance.

Types of black inclusions

Some of the most common black crystal inclusions that can give diamonds a black appearance include:

  • Pyrolytic carbon – Small black flecks made of carbon
  • Prongsite/Bort-type carbon – Black or dark green cubic crystal inclusions
  • Graphite – Black carbon crystal inclusions
  • Polycrystalline diamond – Clusters of many tiny black diamond crystals

These black carbon inclusions absorb and scatter light within the diamond, causing its dark body color. But importantly, these inclusions are still carbon, so they do not interfere with the diamond’s thermal or electrical conductivity.

How inclusions affect color

With most diamonds, inclusions decrease clarity rather than change color. But in black diamonds, the high concentration of tiny black inclusions absorbs light and makes the diamond appear black. Without these inclusions, black diamonds would be transparent and colorless just like other diamonds.

However, almost all natural diamonds have some inclusions, even very small ones only visible under magnification. Black diamonds are unique in that their pronounced black inclusions cause an obvious change in appearance while still allowing the diamond to test as genuine.

Where do black diamonds come from?

Black diamonds originate from very ancient diamond mines located in Brazil and Central Africa. The two main sources are:

  • Brazil – Diamantina, Bahia, Minas Gerais
  • Central African Republic – West African Craton mines

These ancient diamond mines formed under unique conditions over 3 billion years ago that facilitated the development of black diamond inclusions:

Primitive carbon sources

The carbon sources that were crystallized into diamond under high pressure had more primitive forms of carbon present, like pyrolytic carbon and graphite. These carbon types formed the black inclusions rather than transparent diamond.

Long crystallization periods

Diamonds formed over very long periods of up to 2.9 billion years. This allowed time for the black carbon crystals to be incorporated into the growing diamond lattice.

Higher temperatures

Higher temperatures in these mines enabled the black carbon flecks to become embedded in diamond growth. Temperatures reached over 1,650°C (3,000°F) in Central African mines.

These ancient conditions produced diamonds with dense concentrations of black carbon inclusions, giving them their opaque, dark coloration.

Are black diamonds more expensive?

Yes, black diamonds are considerably more expensive than colorless diamonds. There are two main reasons driving the high prices of black diamonds:

  1. Rarity – Black diamonds are much rarer than clear or colored diamonds. They comprise less than 1% of diamonds mined worldwide.
  2. Demand – There is high demand for the exotic look of black diamonds, especially for jewelry. Their striking appearance commands premium prices.

On average, black diamonds will cost around 30-50% more per carat compared to an equivalent clarity colorless diamond. For very high clarity black diamonds, prices can be doubled or tripled.

Price factors

Like other diamonds, main factors affecting a black diamond’s price per carat include:

  • Carat – Larger carat weights fetch exponentially higher prices.
  • Clarity – The least inclusions increase values dramatically.
  • Cut – Quality cutting optimizes black color appearance.
  • Origin – Central African diamonds cost more.

But carat weight has the most significant impact, with black diamond prices rising rapidly for carats above 2.0. A 5 carat high clarity black diamond could easily cost over $100,000.

Price trends

In recent decades, prices for natural black diamonds have consistently trended upwards as they gain popularity in jewelry markets. Their mystique and sophistication have made them highly coveted gemstones.

This strong price growth is expected to continue into the future as black diamond supply remains very limited. Black diamonds are likely to become increasingly valuable over the next 5-10 years.

How can you tell if a black diamond is real?

There are a few techniques that can help determine if a black diamond is real or a simulant:

  1. Diamond tester – Use a thermal probe or electrical resistance tester, as described above.
  2. Jeweler inspection – Experienced jewelers can identify natural vs synthetic black diamonds.
  3. Certification – GIA or other gem lab certification can authenticate black diamonds.
  4. Microscope – Inclusions only seen in natural black diamonds can be observed.

A diamond tester provides the quickest and most accessible method to test at home. Natural black diamonds will pass diamond tester probes. But note that treated black diamonds that have been artificially coated or irradiated may still pass a diamond tester, so other methods may be needed to confirm completely natural origin.

Spotting simulants

Some simulants and treatments that may mimic black diamonds include:

  • Black cubic zirconia
  • Treated or coated colorless diamonds
  • Diamonds artificially irradiated to black
  • Synthetic black diamonds – very rare and expensive currently

Cubic zirconia and other diamond simulants will fail diamond testing. Coated/treated diamonds are harder to identify, but an accredited gem lab can spot these through advanced testing.

How to choose a diamond tester

For testing black diamonds at home, select a robust, electronic thermal conductivity tester with precision sensors. Models with both thermal probe and electrical resistance testing are ideal for confirming natural black diamonds. Some top handheld testers include:

  • Presidium Duotester
  • GIA Diamond Check
  • De Beers DiamondView

Avoid older thermal testers based on metal tips touched to stone. Electronic testers with digital readouts offer far more accuracy.

Are black diamond engagement rings a good choice?

Black diamond engagement rings are a beautiful, unique choice. However, there are some considerations for selecting black diamonds:


  • Striking, elegant appearance
  • Rare and distinctive
  • Trendy, alternative look

Potential cons

  • More expensive than clear diamonds
  • Durability concerns if poor clarity
  • Limited style/setting options

Ensure black diamonds have decent clarity ratings for engagement rings that will be worn daily. Inclusions impact structural integrity if they are too pronounced. Also research jewelry settings – black diamonds show off best in settings with yellow gold or platinum.

Increasing popularity

Black diamond engagement rings have surged in popularity over the past decade. Their distinctive beauty and mystique make them a dramatic choice. For fashion-forward couples looking for something nontraditional, black diamonds present an exciting alternative to standard white diamonds.

It is important to note that black diamonds are considered higher maintenance. Their dark tones can show oils, dirt, and scratches more visibly. Proper care is essential, so black diamonds are best for responsible wearers.


Black diamonds possess the same exceptional thermal and electrical conductivity properties as colorless and fancy colored diamonds. This allows them to pass diamond tester identification. Their dark black appearance comes from microscopic black carbon inclusions in the diamond crystal structure. These black imperfections are naturally formed during ancient diamond creation under high heat and pressure conditions.

While quite rare, making up under 1% of natural diamonds, black diamonds test as genuine diamonds. When purchasing, especially for important jewelry pieces, it is still worthwhile to authenticate them through lab certification and expert inspection along with diamond tester probing.

The exotic black color, opacity, and rarity mean black diamonds demand a significant premium price over conventional clear diamonds. Their unusual beauty and mystique make them a compelling choice for luxury jewelry and engagement rings. As supplies remain tight, natural black diamonds are poised for strong price appreciation going forward.

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