Gemologist Guide to Identifying Pyrite

Pyrite is commonly known as “Fools Gold” because its color is similar to actual gold. The interesting thing about pyrite is it comes in different shapes and sizes. The general population is unaware of the pyrite sunbursts and perfect crystal squares. Over the years it has gained in popularity due to the unique characteristics mentioned above.

Pyrite or iron pyrite is an iron sulfide that forms naturally in pyritohedra or cubes with 12 five-sided faces. It’s found in sedimentary, igneous, and metamorphic rock.

If you’re worried about mistaking pyrite for gold then don’t worry because you can easily distinguish between the two by implementing the testing procedures below.

 

How to Identify Pyrite Through Testing

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

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

 

 

Visual Inspection

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

 

 

Is it a cabochon? If you’re dealing with a cabochon, it should have a high polish with some pitting and fissures. These are not common but can be cut with ease and you might find them in custom jewelry. More than likely they’ll be cut in standard shapes and sizes because the material is readily available.

 

 

Is it faceted? While pyrite can be faceted, it’s not something we commonly see as it’s often used for beads and cut en cabochon. Faceted pyrite will offer a bit of sheen but looks more like brass in this form.

 

 

Is it a specimen? Pyrite is found in different forms. You’ll better identify these forms by looking at and inspecting this mineral over time. Here’s a list of characteristics pyrite displays when it’s a specimen.

• Can resemble a gold nugget.
• Some specimens will contain cube blocks.
• Some specimens may have a light silver sheen over brass.

 

 

Is it tumbled? Very common to find tumbled pyrite.  Again, there will be a nice polish with some tumbled pieces having a more golden color while others are more brass-like.

 

Physical Properties of Pyrite

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

 

Color:

Clarity / Transparency: Opaque

Luster: Metallic

Cleavage: Breaks with a conchoidal fracture

Fracture: Slightly uneven and 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 use destructive tests.

A mineral streak test is when you scrape the stone against a harder surface to see what color remains. Pyrite has a greenish-black to brownish-black.

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

 

Magnet Test

Pyrite is not magnetic, so it shouldn’t respond to common magnets.

 

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.  That said, pyrite has a hardness of 6 to 6.5 on the Mohs hardness scale.

 

Refractive Index Test

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

 

Pyrite’s Refractive Index: Cannot Be Determined

 

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.

 

Occasionally, 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 pyrite, 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: None

 

Single or Double Refraction

You will not use this test for pyrite, but I’ll include the instructions, so you know how it’s done.

For this test to be accurate and beneficial, the stone must be transparent.  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 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. A small amount of distortion might occur when they’re thick, but light will pass through them relatively freely.

Pyrite is opaque.

 

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.

 

Pyrite’s Specific Gravity: 4.9 to 5.2

 

As helpful as specific gravity is for identifying minerals, amateurs are usually constrained by the need for more 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, 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 pyrite 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.

Feel free to reach out if you run into any issues or need clarification. I’ll do my best to assist you in the identification process.