Gemologist Guide to Identifying Zircon

Zircon is a well-known gemstone because it’s commercially available in jewelry and sought after by gem cutters.  You’ll typically see it in blue to bluish-green hues and sizes range from just under a carat up to 12 carats plus.

What you probably don’t know about Zircon is the color of the gemstone when it is mined is actually brown to yellowish-brown.  The miners take the gem rough and heat it up to remove the browns and the blue color you’re most familiar with appears.  Even though, this treatment is enhancing the gemstone it does not negatively affect the price.

 

How to Identify Zircon 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, feel free to share them with us.

Let’s take a deeper look into how to identify Zircon as a pro.

 

 

Visual Inspection

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

Is it gem rough? If you have a single crystal or parcel of gem rough and need to confirm they’re authentic Zircon then you’ll want to look for double refraction of light when you rotate the gem rough.  Crystal structure should match up with Zircon.  If the crystal structure is off then you might have Garnet or Topaz.

Blue Zircon will have a blue-green hue with blue flashes unlike Topaz which will have a solid blue hue throughout the stone.

If you’re looking at raw material with no heat treatment then it will be champagne to brown in color and sometimes you can see flashes of red. These are typically tumbled or alluvial in nature.

To become really good at identifying rough Zircon you’ll need to spend time at rock and mineral shows viewing the rough and talking to dealers.

Is it faceted? A finished stone is much easier to identify because the double refraction inside the Zircon is very apparent.  You’ll want to hold the stone with tweezers or in between your fingers and slowly rotate it.  You’ll notice the edges of the facets will converge and diverge.  This is a leading indicator you’re looking at authentic Zircon.

 

Physical Properties of Zircon

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

Color: Dark Brown, Black, Light Brown, Gray, Brownish-Red, Pink, Orange, Yellow, Light Blue, Light Green, Light Purple, White, Colorless

Clarity / Transparency: Transparent to opaque

Luster: Greasy to Adamantine – Radioactive Zircon has a Pitchy Luster

Cleavage: 3,2

Fracture: Conchoidal to Uneven

 

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 involves scraping the stone against a harder surface to see what color remains.

Tumbled specimens are tested by scraping a specimen across a piece of ungalvanized porcelain, typically known as a streak plate.

Zircons create a colorless streak, meaning the test will be pointless and irrelevant.

 

Magnet Test

Zircon is not magnetic, so it shouldn’t respond to common magnets. However, there are exceptions to the magnet test.

For instance, the crystals themselves aren’t magnetic when it comes to Zircon stones and magnetism. However, they instead contain magnetic minerals and other elements. Which suggests that they were magnetized by a planetary magnetic field. Magnetite retains magnetism for long periods, and while Zircon is still in the earth, magnetite can gather in the holes or pores of Zircon.

 

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, Zircon has a hardness of 7.5 on the Mohs hardness scale.

 

Refractive Index Test

Determining the refractive index, or RI as it’s referred to by gemologists, for Zircon is relatively 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.

 

Zircon’s Refractive Index: Low: 1.78 to 1.815  Intermediate: 1.83 to 1.93  High: 1.92 to 1.94

 

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 the gauge.

 

Birefringence Test

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: 0 – 0.008 (Low) 0.008 – 0.043 (Intermediate) 0.036 – 0.059 (High)

 

Single or Double Refraction

Zircon has a double refraction. You’ll see twice as much fire and twice as many facets during the test.

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 remains light, it is likely AGG. If the stone’s lightness or darkness changes, DR is likely.

 

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.

Zircon has a transparent to opaque diaphaneity. However, its transparency depends on the form it has taken. If the Zircon has more of an earthy form, there won’t be as much light traveling through it. Still, if it happens to take on a crystalline structure, you should expect an opaque diaphaneity.

 

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.

 

Zircon’s Specific Gravity: 4.6 – 4.8

 

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 of identifying a piece of Zircon 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 get confused, feel free to reach out, and I’ll do my best to assist you in the identification process.