Gemologist Guide to Identifying Almandine Garnet

Almandine garnets can be challenging to distinguish from other garnet types because they’re just red and the gemstone market is flooded with them.  These beautiful gemstones usually have darker tones when compared to other red garnets. However, top-quality specimens have red colors that are similar to that of a ruby.

 

How to Identify Almandine 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 gems, then feel free to share it with us.

You’ll be relying heavily on the visual inspection as well as the refractive index test to help you distinguish the specific garnet variety.

Let’s look deeper into how to identify Almandine garnet like a pro.

 

 

Visual Inspection

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

Almandine garnets can be found in various faceted styles and shapes, including oval, round, heart, pear, trillion faceted, and cabochons.

 

Is it a cabochon? When dealing with a cabochon, it should have a medium to high polish with pitting on the surface.  Almandine garnets are commercially available in cabochon form or you’ll see almandine garnet crystals with the faces of the crystal polished with a cabbing machine or diamond wheel.  These crystals are very dark-red to almost black looking.

 

Is it faceted? If you have a faceted piece of almandine garnet, it should be transparent and have a dark red color.  When you’re looking at the faceted stone you’ll notice flashes of red but the predominant color reflected back to your eye will be black. The light entering the stone is unable to be reflected back which produces dark spots in the finished gemstone. Another identifying characteristic is the depth of the faceted gemstone.  Almandine garnets will be shallow because the cutter wants the stone to show as much red as possible when viewing it from the crown.

 

Is it a specimen? Almandine garnet is commonly found and displayed in crystal form or on matrix. You’ll get better at identifying these forms by looking at and inspecting this mineral over time. Here’s a list of characteristics almandine garnet displays when it’s a specimen.

 

• Deep, dark red colors with hints of black and sometimes slight shades of yellow.
• You’ll be able to do the streak test; keep reading below if you have a rough piece with no commercial value.
• An iridescence effect on the outside surface.  This will look like a rainbow of colors, almost like a thin coating.
• It can be found in crystal form and they’ll range in size.

 

Is it tumbled? Very common to find tumbled almandine garnet and you might be lucky enough to come across some with asterism, a star-like effect.  The polished surface will take a medium to high polish with some pitting.  You’ll notice that many tumbled specimens seem to have more black and dark red, with some containing hues of dark brown.

 

Physical Properties of Almandine Garnet

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

 

Color: Red, Reddish-Orange, Purplish-Red, Dark Brownish-Red

Clarity / Transparency: Transparent to Opaque

Luster: Vitreous

Cleavage: None

Fracture: 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 involves scraping the stone against a harder surface to see what color remains.  When dealing with almandine garnet, you’ll notice a red or reddish-brown streak.

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

 

Magnet Test

Almandine garnet is magnetic, and they show a response to N52 magnets or neodymium magnets. These gems are more magnetic than other transparent gems because they generally contain higher concentrations of paramagnetic iron and manganese.

Different garnets containing different chemical compositions can also differ in their magnetic susceptibility. For instance, spessartine garnets are the most magnetic, while tsavorite garnets have the lowest magnetic attraction.

 

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, almandine garnet has a hardness of 6.5 – 7.5 on the Mohs hardness scale.

 

Refractive Index Test

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

 

Almandine Garnet’s Refractive Index: 1.770 – 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 the gauge.

 

Birefringence Test

You won’t be using this test for almandine 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

Almandine garnet is singly refractive.

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.

Almandine garnet is transparent to subtransparent to translucent. However, its translucency depends on the form it has taken. If the almandine garnet has a dark crystal form, there won’t be much light traveling through it. Still, if it happens to take on a crystalline structure, you should expect more of 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.

 

Almandine Garnet’s Specific Gravity: 4.05

 

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 almandine 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 run into any issues or you need clarification, then feel free to reach out, and I’ll do my best to assist you in the identification process.