Gemologist Guide to Identifying Spessartite Garnet

Spessartite has a beautiful orange color, but iron impurities are typically present, giving the gemstone a reddish-orange or brownish-orange color instead. While brownish, reddish, and yellowish tints of Spessartite are frequent, pure orange is not. The most desirable color of this stone is a fiery red with slight orange tints.

Spessartite garnets can be identified by their color. Garnets, in general, can be distinguished from other gems by their occurrence in metamorphic rocks, their hardness, refractive index, color, and crystal structure. You may also see spessartite garnets on the market as Mandarin Spessartite; they’re the same gem.

 

How to Identify Spessartite Garnet 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 spessartite garnet like a pro.

 

 

Visual Inspection

The visual inspection starts with what form of spessartite garnet 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 medium to high polish with very little pitting on the surface.  Inclusions will be common and the bottom of the cab should be polished.

 

Is it faceted? If you have a faceted piece of spessartite garnet, then it will be transparent with orange and yellow flashes of color.  Large stones can be obtained but they’re rarer and the cost per carat goes up considerably. Most stones will fall between 1 carat and 5 carats in size. A flawless stone is rare so expect to see some small to light inclusions.

 

Is it a specimen? Spessartite Garnet 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 spessartite garnet displays when it’s a specimen.

 

• Bright to dark orange colors with light brown and orange-brown undertones.
• It can be found in crystal form, but you’ll find water-worn nodules are more common.

 

Is it tumbled? These are not very common and if you did find them then they should be heavily included and of low quality.  I would confirm identity with the refractive index test because you might be confusing spessartite with citrine or yellow glass.

 

Physical Properties of Spessartite Garnet

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

 

Color: Orange, Orangey-Yellow, Red-Brown

Clarity / Transparency: Transparent – Translucent

Luster: Vitreous

Cleavage: Usually None

Fracture: Sub-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.

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

 

Magnet Test

Spessartite garnet is magnetic, so it will likely respond to neodymium magnets.

It’s entirely normal for spessartite garnet to contain high concentrations of iron and/or manganese. Spessartite garnets can be distinguished from other garnet varieties by their high range of magnetic susceptibility (SI).

 

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, Spessartite Garnet has a hardness of 6.5 to 7.5 on the Mohs hardness scale.

 

Refractive Index Test

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

 

Spessartite Garnet’s Refractive Index: 1.790 – 1.820

 

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 spessartite garnet, 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 spessartite garnet, 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 to 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.

Spessartite garnet is translucent. However, its translucency depends on the form it has taken. If the Spessartite Garnet 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.

 

Spessartite Garnet’s Specific Gravity: 4.12 – 4.20

 

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 spessartite garnet 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’ve determined your garnet is not spessartite then you should check out “how to identify almandine garnet” to determine if it’s a different variety.