Gemologist Guide to Identifying Sphalerite

Sphalerite is not well known among mainstream gemstone buyers but serious collectors know about its unique characteristics.  Due to its high dispersion and glowing colors, Sphalerite is sought after by rare gemstone connoisseurs.

Sphalerite is a zinc sulfide mineral with the chemical composition (Zn, Fe)S. It can be sourced from every continent but rarely makes its way into jewelry due to its low score on the hardness scale.

 

How to Identify Sphalerite 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 Sphalerite like a pro.

 

 

Visual Inspection

The visual inspection starts with what form of Sphalerite you have. The questions below are fairly easy to answer but each type will have its own process for identifying them.
The trick to visually identifying Sphalerite is high dispersion, that is the tell! Sphalerite has a very high dispersion, similar to Diamonds. When looking at the stone you’ll notice the rays of light being reflected back to your eye will have a rainbow-like appearance.

 

Physical Properties of Sphalerite

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

Color: Yellow, Light to Dark Brown, Black, Reddish Brown, light Blue, Pale Green, Greenish Yellow, Colorless

Clarity / Transparency: Transparent – Translucent

Luster: Adamantine, Resinous, Greasy

Cleavage: Perfect Dodecahedral on [011]

Fracture: Uneven to Conchoidal

 

Magnet Test

Sphalerite is not magnetic, so it shouldn’t respond to common magnets. However, there are exceptions to the magnet test.
For instance, if there are impurities in the Sphalerite, there is a very slight chance that it may respond weakly to a magnet. However, the probability of that is rather unlikely.

 

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, Sphalerite has a hardness of 3.5 to 4 on the Mohs hardness scale.

 

Refractive Index Test

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

 

Sphalerite’s Refractive Index: 2.369

 

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 Sphalerite 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, but may show anomalous birefringence due to strain.

 

Single or Double Refraction

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 appeared 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.

Sphalerite has a single refraction

 

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.

Sphalerite is transparent to translucent. However, its translucency depends on whether it’s gem quality or specimen grade. However, when the specimen is iron-rich it will have 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.

 

Sphalerite’s Specific Gravity: 3.9 – 4.1

 

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 Sphalerite 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.