Gemologist Guide to Identifying Diamonds

Having to identify a diamond is more common than you’d imagine because people inherit jewelry or buy jewelry from an auction or garage sale and believe they have a valuable gemstone. The most common outcome is the owner realizing they have cubic zirconia or some type of imitation.

That said, let’s look into how to identify diamonds. After all, there are a lot of fakes out there, and it’s always good to know you’ve got the real deal.


How to Identify Diamonds Through Testing

There are various ways to identify rocks, minerals, crystals, and gemstones, but we will be using a method I learned while attending the Gemological Institute of America.  If you’ve learned a unique way to identify gemstones, then feel free to share it with us.

Let’s take a deeper look into how to identify Diamonds like a pro.


gemstone testing lab


Visual Inspection

The visual inspection starts with what form of Diamond you have.  The questions below are fairly easy to answer, but each type will have its own process for identifying them.


Is it a cabochon? While the polished cabochon is an extremely popular cutting style in colored stones, it’s rarely attempted with diamonds due to its hardness.


Is it faceted? A facet refers to the flat surfaces of a gemstone that are arranged in geometrical patterns. When this gemstone is cut by a professional, a diamond’s facets will cause stunning optical effects on its surface and additional sparkle as the facets interact with light and one another.

You’ll want to have a pair of tweezers and a 10x loop when dealing with a faceted diamond.  Hold the loop 1/2 inch away from the stone and you’ll start to see a detailed view of the diamond’s interior.  You want to look for feathers, included crystals, and the occasional cloud.  Diamonds have very distinct inclusions that help you identify them.

To get really good at identifying Diamonds you’ll need to spend the majority of your time viewing them through a 10x loop or microscope.  This will build your knowledge bank on common characteristics.

Pro tip: If the girdle is not faceted then it should have a waxy look to it.  This will be another leading indicator that the diamond is authentic.


Is it a specimen? Many people only recognize diamonds that have been cut and polished. Raw diamonds, while popular, don’t get as much of the limelight. Diamonds cut with more proportions have more brilliance. Here’s a list of characteristics Diamonds display when they’re a specimen.

  • Transparent with brownish or yellowish tints.
  • You will not be able to do a streak test on diamonds because they are too hard and don’t streak. However, due to its hardness, a diamond will produce a scratch on the porcelain streak plate.
  • An iridescence effect on the outside surface. However, the sparkle is more of a gray color rather than a rainbow. If you see a rainbow, your specimen is likely not a diamond.
  • Diamonds found raw occur most often as euhedral or rounded octahedra and macules or twinned octahedra. The structure has a cubic arrangement.


fancy cut diamond


Physical Properties of Diamonds

Let’s take a look at the physical properties of diamonds. Knowing what to look for will help you more easily identify what you’re looking at.

Color: Pale Yellows, Grays, Browns, White, Black, Blue, Greenish, Reddish, Colorless

Clarity / Transparency: Transparent to Translucent (in rough specimens)

Luster: Adamantine to Waxy

Cleavage: Perfect in Four Directions Forming An Octahedron

Fracture: Conchoidal


The Streak Test

This is a destructive test so you need to ensure that you’re allowed to damage the specimen or stone if you choose to use this method.  Once you’ve developed robust knowledge in identifying rocks and minerals, you won’t be using destructive tests.

A mineral streak test is when you scrape the stone against a harder surface to see what color remains.  When dealing with Diamonds, you’ll notice that a streak test doesn’t work.

If you conduct a streak test with what you think is a diamond, you’ll notice it cuts or scratches the plate.


Magnet Test

Real diamonds are not magnetic, so they shouldn’t respond to common magnets. However, there are exceptions to the magnet test. If your diamond is attracted to a magnet, it’s definitely synthetic.


Hardness Test

I don’t recommend actively testing the hardness of a stone because it’s destructive in nature and doesn’t really provide a definite answer to what type of stone it is.  With that being said, Diamonds have a hardness of 10 on the Mohs hardness scale.


Refractive Index Test

Determining the refractive index, or RI as it’s referred to by gemologists, for diamonds is fairly straightforward but you’ll need a specific piece of test equipment and the RI fluid to go with it.  Before you place the stone on the refractometer you want to make sure you have a flat, somewhat polished, surface to take a reading from.


Diamond’s Refractive Index: 2.417 – 2.419


Each gemstone has its own RI, so discovering a sample’s RI can help you figure out what sort of stone it actually is.


Step 1 – Place a small bead of RI fluid on the metal surface of the refractometer near the back of the crystal hemicylinder (the window on which the stone will sit).

Step 2 – Place the stone facet face down on the fluid dot and slide it toward the middle of the hemicylinder crystal using your fingers.

Step 3 – Look through the viewer lens without magnification. Continue looking until you see the outline of a bubble, then look at the bottom of this bubble. Take the reading from there, rounding the decimal to the nearest hundredth.

From time to time, you’ll run into the issue of not having a flat surface to work with. In this instance, you’ll need to leave the top of the refractometer open and hold the rounded stone with your hand.  Hopefully, you’ll be able to pull a reading off of the gauge.


Birefringence Test

You won’t be using this test for Diamonds but I wanted to include this test just in case you were considering it in your process.

Consider testing the birefringence, as well. Birefringence is related to RI. While doing the birefringence test, you will turn the gemstone on the refractometer six times throughout the observation period and note the changes.

Perform a standard RI test. Instead of keeping the stone still, gradually turn it 180 degrees, making each separate turn about 30 degrees. At each 30-degree mark, take a new RI reading.

Subtract the lowest reading from the highest to find the stone’s birefringence. Round it to the nearest thousandth.


Birefringence: N/A


Single or Double Refraction

Diamonds are singly refractive. For this test to be accurate and beneficial, the stone needs to be transparent in nature.  If the light won’t pass through the stone, then there is no way to test for single or double refraction.

Check for single or double refraction. Use this test on translucent and transparent stones. You can determine whether the stone is only singly refractive (SR) or doubly refractive (DR) to help identify it. Some stones can also be classified as aggregate (AGG).

Turn on the light of a polariscope and place the stone face down on the lower glass lens (polarizer). Look through the top lens (analyzer), turning the top lens until the area around the stone looks darkest. This is your starting point.

Turn the analyzer 360 degrees and watch how the light around the stone changes.

If the stone appears dark and stays dark, it is likely an SR. If the stone starts light and stays light, it is likely AGG. If the lightness or darkness of the stone changes, it is likely DR.


Checking The Diaphaneity

Diaphaneity refers to the mineral’s ability to transmit light. For instance, some minerals are transparent or translucent. When they’re thick, a small amount of distortion might occur, but light will pass through them relatively freely.

Diamonds are transparent to subtransparent to translucent.


Finding The Specific Gravity

Every stone has its unique specific gravity, which helps us identify them. Specific gravity is one of the best properties to measure when identifying mineral specimens. Most minerals have a narrow range of specific gravity, so getting an accurate measurement can go a long way toward identification.

Specific gravity is a unitless number describing how heavy a mineral is compared to equal volumes of water. For example, if a mineral is three times as dense as water, it’ll have a specific gravity of three. This is useful because while two minerals might be the same size, they’ll each have a different specific gravity.

The larger the sample, the more precise the readings tend to be. Remember that this technique can only be used for single mineral or crystal masses. It will not work for minerals embedded in host rocks.


Diamond’s Specific Gravity: 3.52


As helpful as specific gravity is for identifying minerals, amateurs are usually constrained by the lack of necessary tools for the job. However, one way to work around this is to hold the specimen and note how heavy or heft it feels compared to what you might expect a specimen of that size to weigh.

If you want to determine the specific gravity of your stone like a pro, then you’ll need to invest in a higher-end scale.  This is the one gemologists use OHAUS Density Determination Kit


Identifying Rocks and Minerals Like a Pro

Hopefully, you feel confident in your practice to identify a piece of Diamond after reading and applying this guide.  You’ll be using the visual part of this guide the most, and you’ll get better as you interact with more gemstones.  Before you know it, you’ll be identifying stones like a gemologist.

If you run into any issues or you get confused then feel free to reach out, and I’ll do my best with assisting you in the identification process.

Jerred Morris
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