Gemologist Guide to Identifying Demantoid Garnet

Demantoid Garnets are a rare variety of Andradite Garnets and you probably won’t come across one of these unless you’re actively identifying and assessing gemstones on a regular basis.  Gemstone collectors are the biggest buyer of this species of Garnet and they’re not commercially available.

This precious gemstone’s name means “diamond-like” which comes from the Dutch, making reference to the stone’s outstanding quality.

 

How to Identify Demantoid 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 Demantoid Garnet like a pro.

 

Visual Inspection

The visual inspection starts with what form of Demantoid 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? Demantoid Garnets are not often seen with a cabochon cut. However, these gems can be cut into almost any shape, one of the most popular being diamond-shaped.

Is it a faceted round brilliant cut? Brilliant cut stones consist of kite-shaped and triangular facets. A brilliant-cut gives off the most scintillation of any other cut.

Is it a faceted cushion cut? If you have a cushion cut Demantoid Garnet, it should have a square or rectangular shape with rounded edges. It will slightly resemble a pillow, hence the name “cushion cut.” The gemstone will feature modern brilliant faceting.

Demantoid Garnets are a type II clarity stone. They might show surface abrasions. Some stones may contain fibrous chrysotile asbestos or horsetail inclusions, see below.

 

 

Physical Properties of Demantoid Garnet

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

Color: Light to Deep Green, Yellowish Green to Golden Brownish Green

Clarity / Transparency: Transparent to Opaque

Luster: SubAdamantine, 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 Demantoid Garnet, you’ll notice it doesn’t have a streak, and if it does happen to leave any color, it will be white.

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

Due to Demantoid Garnet’s rarity, I highly suggest never using the streak test, whether it is raw or rough, and especially not if it’s polished.

 

Magnet Test

Demantoid Garnet is actually slightly magnetic, so it responds to an N52 magnet. Garnets are more magnetic than most other transparent gems because they typically contain higher concentrations of paramagnetic iron and manganese.

 

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, Demantoid Garnet has a hardness of 6.5 to 7 on the Mohs hardness scale.

 

Refractive Index Test

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

 

Demantoid Garnet’s Refractive Index: 1.870 to 1.890

 

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 Demantoid 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 Demantoid Garnet, but I wanted to include this test just in case you were considering it in your process. However, these gemstones have a single refraction with an index of 1.88 to 1.94. 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.

Demantoid Garnet is transparent to translucent. However, its translucency depends on the form it has taken. If the Demantoid 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 a transparent or translucent 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.

 

Demantoid Garnet’s Specific Gravity: 3.810 – 3.890 (usually 3.840)

 

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