Gemologist Guide to Identifying Spinel

Spinel is a highly sought-after gemstone and can be found in high-end jewelry. Unfortunately, it doesn’t have any commercial use outside of the precious gemstone trade and that is why it’s not well known. It has all of the characteristics to make for an amazing gemstone to be worn by the masses but for some reason, it has never taken off.

Spinel is a gemstone that can be red, blue, purple, rose, black, or brown in color, or also colorless. Its red and blue varieties are sometimes confused with ruby or sapphire, two of the gemstone variations of the mineral corundum, because of their striking visual resemblance. The main sources of high-quality spinel come from Mahenge, Myanmar, Sri Lanka, Tanzania, and Tajikistan.

 

How to Identify Spinel 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 spinel like a pro.

 

 

Visual Inspection

The visual inspection starts with what form of Spinel 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? If you’re dealing with a cabochon, then it should have a medium to high polish with little to no pitting on the surface.  Because blue and red Spinel is rare, the colors are usually pink, green, yellow, and orange.

 

 

Is it faceted? If you have a faceted piece of spinel, then you’ll need a 10x-powered loop to view the inside of the stone. Most spinels have small inclusions which will assist you in identifying a real one from a synthetic one. Most of the synthetics will have metallic inclusions or the stone will be flawless.

 

 

Is it a specimen? Spinel is found in different forms, and you’ll get better at identifying these forms by looking at and inspecting this mineral over time. Here’s a list of characteristics Spinel displays when it’s a specimen.

• Pink, magenta, or violet with red undertones.
• You’ll be able to do the streak test. Keep reading below if you have a rough piece with no commercial value.
• It can be found in crystal form, but it’s rarer, and you’re not likely to come across it very often.  These crystals are typically on the smaller side and considered to be micro-specimens.

 

Is it tumbled? Very uncommon to find tumbled spinel due to its value and size. Most spinel range in size from 1-10 grams which doesn’t make for great tumbling material.

 

Physical Properties of Spinel

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

 

Color: Red, Orange, Yellow, Pink, Blue, Violet, Brown, Colorless, Gray, Black, Green

Clarity / Transparency: Transparent to Opaque

Luster: Vitreous

Cleavage: None

Fracture: Conchoidal, 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 is when you scrape the stone against a harder surface to see what color remains.  When dealing with Spinel, you’ll notice a white streak.

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

 

Magnet Test

There are various species of Spinel, including aluminum, iron, chromium, cobalt, and vanadium. All species containing iron are highly magnetic. Magnetite is an opaque black Spinel and acts as a natural magnet. This stone may have the highest magnetic susceptibility of any naturally occurring mineral.

That said, depending on Spinel’s chemistry, the magnetic attraction can be weak to incredibly high.

 

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, Spinel has a hardness of 8 on the Mohs hardness scale.

 

Refractive Index Test

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

 

Spinel’s Refractive Index: 1.71 – 1.73

 

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

Spinel is transparent to opaque. However, its transparency depends on the form it has taken. If the Spinel has an earthy form, there won’t be 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.

 

Spinel’s Specific Gravity: 3.54 – 3.63

 

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 spinel 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, then feel free to reach out, and I’ll do my best to assist you in the identification process.