Gemologist Guide to Identifying Andalusite

Andalusite is a strongly pleochroic gemstone and is highly saught after by lapidary artists because of this unique trait. Pleochroic gems can display different colors when they’re viewed from different directions. Expert cutters can produce beautiful faceted stones that display the gem’s pleochroic colors in the face-up position.

These gemstones are most commonly found in Gneiss, Schist, and Hornfels but some mines have discovered this material in Granite and Granitic Pegamite.

One of its most important uses is making heat-resistant bricks and ceramic objects used in steel-making and metal-processing industries. Andalusite is also used to make glass, chemicals, and various ceramic products. Exceptional crystal specimens are valued by mineral collectors, and specimens boasting spectacular clarity and color are cut into gemstones.


How to Identify Andalusite Through Testing

There are various ways to identify rocks, minerals, crystals, and gemstones, but we will use a method I learned while attending the Gemological Institute of America.  If you’ve learned a unique way to identify gems, please share it with us.

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


Visual Inspection

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


andalusite cabochons


Is it a cabochon? A cabochon should have a medium to high polish with little to no pitting. You might expect some inclusions that occur naturally in the gemstone. You’ll be looking for the pleochroic phenomenon when viewing the cabochons. You’ll typically see yellows, browns, and greyish-green.


faceted andalusite


Is it faceted? If you have a faceted piece of Andalusite, it should be transparent. Once again the pleochroic properties will be the biggest tell that you have Andalusite. With a 10x loop you can look for natural inclusions to determine if the stone is natural or synthetic. Large stones are rare and demand a high price per carat while smaller stones can be sourced from gem dealers.


andalusite crystal rough


Is it a specimen? Andalusite is found in different forms. You’ll better identify these forms by looking at and inspecting this mineral over time. Here’s a list of characteristics Andalusite displays when it’s a specimen.

  • Specimens have somewhat of a rough Amber look about them.
  • Rare specimens can contain cluster crystals in a column form.


Is it tumbled? It is not very common to find tumbled Andalusite but if you do you’ll want to look for the pleochroism in the stones to help you correctly identify it.


Physical Properties of Andalusite

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

Color: Pink to Red Brown, Occasionally, White, Green, Yellow, Gray – rarely Violet

Clarity / Transparency: Transparent to Translucent, sometimes Opaque

Luster: Vitreous, Sub-Vitreous, Greasy

Cleavage: Distinct/Good – Good on [110] Poor on [100]

Fracture: Irregular / Uneven, 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 use destructive tests.

A mineral streak test is when you scrape the stone against a harder surface to see what color remains. Andalusite produces a white streak.

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


Magnet Test

Andalusite can sometimes contain a very weak magnetic attraction. However, it is likely not enough to help positively identify the stone.


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.  Andalusite 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 Andalusite, is relatively straightforward. Still, 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.


Andalusite’s Refractive Index: 1.629 – 1.640


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.


Occasionally, 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 can pull a reading off of the gauge.


Birefringence Test

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: 0.009


Single or Double Refraction

Andalusite has a slight double refraction.

The stone must be transparent for this test to be accurate and beneficial.  If the light won’t pass through the stone, 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 out light and remains 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. A small amount of distortion might occur when they’re thick, but light will pass through them relatively freely.

Andalusite is translucent to transparent and sometimes opaque.


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.


Andalusite’s Specific Gravity: 3.13 – 3.31


As helpful as specific gravity is for identifying minerals, amateurs are usually constrained by the need for more 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, you’ll need to invest in a higher-end scale.  The OHAUS Density Determination Kit is the one gemologists use.



Identifying Rocks and Minerals Like a Pro

Hopefully, you feel confident in your practice of identifying a piece of Andalusite 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.

Feel free to reach out if you encounter any issues or need clarification. I’ll do my best to assist you in the identification process.

Jerred Morris
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