How to Tell If a Black Onyx Is Real

Although there are many different kinds of black gemstones, few can compare to the cool bohemian vibe of black onyx jewelry. Black onyx has a fascinating history, making it perhaps the most desirable black gemstone of all time. Although black diamonds are only just becoming popular in jewelry, carved black onyx jewelry has had considerable popularity around the world, dating back to Ancient Egypt.

The history of black onyx jewelry is rich and noteworthy. In Ancient Greece and Rome, professional craftsmen would carve cameos and intaglios of meaningful pictures—such as historical people, animals, personal symbols, etc—into the gemstone and create onyx rings, amulets, etc. In centuries to come, popular black onyx jewelry types would be black onyx signet rings, onyx brooches, onyx mourning jewelry, and Art Deco men’s black onyx rings.

Onyx comes in many colors; black is actually one of the rarest colors of natural onyx. That being said, how can you tell if black onyx is real? How can you tell if a black onyx ring is real or plastic, authentic onyx, or a different type of black gemstone? We’ve put together a guide on how to tell if black onyx is real.

Black Onyx vs. Other Black Gemstones

First of all, what is black onyx? Geologically speaking, black onyx is a banded type of silicate mineral called “chalcedony.”  Black onyx isn’t the only type of banded chalcedony—agate is also a banded type of this mineral, and agate bracelets are a popular type of gemstone bead.

Onyx consists of parallel bands of alternating color (it’s more difficult to tell with black onyx, but it’s still there), composed of the minerals moganite and quartz. The colored bands can be almost any color, but the most common onyx colors are brown and red, white and black, and red and white.

Many gems are falsely described as “onyx”, including "Cave Onyx" and "Mexican Onyx", which despite having black and white bands, aren’t real onyx stones. Onyx is also commonly confused with obsidian, opal, alabaster, and marble.

Types of Synthetic and Simulant Onyx

People have been imitating real black onyx since ancient Roman times, when craftsmen made fake sardonyx by layering thin slabs of black, white, and red stones carefully together.

Today, there are many onyx simulants around, as well as lab-created imitation onyx. That being said, onyx simulants have different crystal structures from real onyx, as well as differences in chemistry.

Common false onyx is made from black devitrified glass (opaque glass with a solid color) as would a true black onyx. and black cubic zirconia. Other false onyx jewelry is also often made from natural black gemstones, such as augite and black spinel. Spinel is actually a good gemstone for jewelry, being more durable and actually more valuable than black onyx. However, black onyx jewelry is far more recognized and in demand, so sellers may still label black spinel as black onyx to try to sell it faster.

Types of Onyx Stones

If you want to know if you’re looking at a real black onyx ring or false onyx, it’s helpful to know the four main types of onyx.

• Black and White Onyx (White layered black onyx): Also called “Arabic onyx” it’s the most famous type of natural onyx, as well as the most popular onyx color variety. Gem stores and jewelry shops may refer to it as “true onyx” or “genuine onyx.”
• Sardonyx (Brown, red, white, and yellow onyx): If the onyx has a reddish color it’s called “sardonyx,” which refers to real onyx with a reddish base color. The top layers of sardonyx stones are typically white, and it may feature alternating bands of whites, browns, or yellow. You can find beautiful sardonyx carvings from ancient times in Europe.
• Nicolo Onyx (Mostly black, thin white layer): This onyx has a cool appearance—its base is black, but it has a thin whitish layer that looks blue/grey in different lighting. It was known to the Egyptians, who used it in sacred artwork.
• Black Onyx (Looks black, hard to see color bands): Natural black onyxes look all black from a distance, but on closer inspection, they feature thin color bands. You may need magnification to see them. While true all black onyxes exist in nature, they are very rare.

Keep in mind that real onyx can vary significantly in appearance. A cut and polished onyx stone will look different than a raw onyx stone. Nicolo onyx may have a strong white layer or a weak blueish layer. Sardonyx can range from bright red to brownish red, to yellowish.

How to Tell if Black Onyx is Real

So, since onyx has been faked and mistaken for millennia, how can you tell if black onyx is real at home? Here are the steps you can take to test black onyx to certify its authenticity:

Step #1: Heat Test

One way to test if black onyx is real is to conduct the heat test or the temperature test.

• Touch the black stone to your face or a hot part of your body. If it feels cool to your skin, it’s a real stone. If it warms up from your body heat, it’s plastic or some kind of synthetic fake.
• Another form of heat test is to check how the stone reacts to higher heat. The melting point of black onyx is over 1000 degrees, which is flame-resistant. Hold the stone over a candle flame or a lighter and see what happens. If it melts or changes shape, it’s plastic black onyx. If it doesn’t, it could be real black onyx—it still might be a different black gemstone, though.

If checking the stone with a flame, be careful not to touch the metal to the flame, or to expose it for too long. You could warp precious metals with the heat and would have to take them in to get repaired.

Step #2: Scratch Test

To test black onyx’s hardness, you can use the scratch test. If your stone is made of plastic, black resin, or obsidian, it will generally be less hard on the Moh’s scale than black onyx.

Like quartz, the hardness of onyx is typically between 6-7 on Moh’s scale of hardness. Materials that are under 6 on this scale will not damage the stone. Harder materials, on the other hand, will scratch it if it’s real black onyx, so you need to be careful conducting this test. Only use safe materials to test the stone on.

• Use one of the following items:
  • Copper coin
  • Your fingernail
  • Standard kitchen knife
  • Stones that are known to be soft, such as:
  • Fluorite
  • Apatite
  • Selenite
• Scratch a part of your black onyx on the material, or scratch the material on the onyx. Choose to test a side of the onyx that is not as decorative, such as the underside.
• Look at both the material and the stone. If it’s black onyx, it should blemish the material, but it should not have a scratch. If the material is left untouched but the onyx is scratched, it could be fake.

Make sure you are scratching the actual stone—if it has a clear sealant on it, this test won’t apply. You will just be damaging the outer coating. Skip this test if your black onyx is coated.

You can also test black onyx through another type of scratch test: Scratch the stone’s surface with a sharp pin. If it removes the color, it’s not real.

Step #2: Shape and Smash Test

Many “black onyx” bracelets are actually made of black plastic, vendors don’t always know this when they’re selling the bracelets. To tell real black onyx bead bracelets from plastic, do the following:

1. Carefully examine the bead. Do you notice any signs that it was made in a factory mold, such as a seam or a bubble? Then it’s a plastic bead or some other type of fake onyx.
2. If you can’t tell if it’s plastic, but don’t care about it if it is, take one of the beads and place it in a small plastic bag and try to smash it with a heavier object with slight force.
3. If the bead breaks into tiny pieces, it’s probably plastic.
4. If the bead breaks apart but doesn’t shatter into tiny pieces, look inside the broken shell. If it’s shiny inside, it’s probably fake—maybe glass or obsidian.

While shiny on the outside, black onyx beads are not polished within, so they should be dull inside. Be careful if doing the smash test, only use one bead so as to not damage the rest of the bracelet in case it’s real black onyx.

Step #3: Acid Test

Real black onyx consists of quartz, which is non-reactive to many acids that are used to clean other rocks. Here’s how to test black onyx with acid to tell if it’s real:

1. Obtain muriatic acid or oxalic acid.
2. Drop the black onyx into the acid and observe the reaction.
3. If you see fizzing/bubbling, it’s not real onyx. Real onyx won’t have much of a reaction to these tests.

Avoid doing this test if you have any cracks in your onyx, if it’s an older/antique onyx jewelry piece, or if it has intricate designs. If there are any cracks, the acid could get inside and mess with the look of the stone, or its metal setting.

Step #4: Visual Inspection

Since we’ve provided information on what real black onyx looks like, you can look closely at your gemstone to see if it resembles real black onyx. This test works better with large black onyx stones—you might not be able to notice with small black onyx jewelry.

1. Check your black onyx for parallel bands. Even if it appears to be a solid color, you may still be able to see some sort of parallel bands running through the stone.
2. If it has bands, it’s probably real black onyx. If it seems uniformly smooth, or has molding elements, it could be fake.

Small black onyx may be highly polished in jewelry, so it can be hard to tell if there is banding or not. You can always take it to a jeweler to certify that it’s real black onyx, or to tell if it’s fake.

Jeweler Tip: Be a Knowledgeable Buyer

There’s a reason black onyx jewelry is more popular than any other type of black gemstone—its dramatic appearance, rich history, and personal meaning it provides are truly unique. Black onyx jewelry for men is really trending right now, so you’re likely to encounter many different varieties of black onyx rings and bracelets for sale.

Before you buy, it’s a good idea to read articles like these, so you’re informed on what you’re getting. Here at ItsHot, we encourage buyers to obtain as much knowledge as they can on diamonds, gemstones, and precious metals. To discover more about gemstone and jewelry education, check out our education section.

Denis Stepansky
Founder of ItsHot.com

Denis Stepansky is a founder of ItsHot, a diamond jewelry and watches store based in NYC. He has been in the jewelry business for about 20 years and owns such high-end jewelry brands as Luccello and Luxurman. As a jewelry expert, he has citations on well-known magazines and newspapers like Insider and Daily Mail.
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