Gemologist Guide to Identifying Tanzanite (Zoisite)

Tanzanite is a variety of zoisites that can be violet, purple, and blue. The different shades of purple and blue you see in tanzanite are due to its strong pleochroism. When valuing tanzanite you’ll learn the most highly saturated stones with a rich blue color are the most sought-after.

These beautiful crystals are sourced from a mine in Tanzania and it is the only known source at this time.

 

How to Identify Tanzanite 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 tanzanite like a pro.

 

 

Visual Inspection

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

 

 

Is it a cabochon? A cabochon should have a high polish with little to no pitting. Inclusions and spots are common for the quality of stone used in cabochons. More than likely they’ll be cut in uniform shapes and on occasion you’ll run into a free from style cab.

 

 

Is it faceted? Faceted tanzanite is commercially available and you’ll find common shapes like round, oval, cushion, and pear. The colors will range from a lilac purple all the way to a deep blue with some stones showing a multitude of colors. Clarity grades should be on the higher side and weights will range from small to large. The pleochroism will be a strong tell for identifying the stone but it doesn’t hurt to use your 10X powered loop to identify some internal inclusions.

 

 

Is it a specimen? Tanzanite 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 Tanzanite displays when it’s a specimen.

• Dark blue, light blue, colorless, and light violet are common colors.
• Inclusions are common.
• Sizes vary from small to medium.
• Some specimens may have shades of gray present.

 

 

Is it tumbled? It’s not very common to find tumbled tanzanite because there’s not a huge market for it. If you attend enough gem and mineral shows then you’re bound to come across it. Tumbled pieces will display a high polish with naturally occurring internal inclusions.

 

Physical Properties of Tanzanite

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

 

Color: Blue, Violet, Bi-color

Clarity / Transparency: Transparent – Translucent

Luster: Vitreous

Cleavage: Perfect in one direction, Good in one direction

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 use destructive tests.

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

Tanzanite is not magnetic, so a magnet test will not help identify this 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.  That said, tanzanite has a hardness of 6.5 on the Mohs hardness scale.

 

Refractive Index Test

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

 

Tanzanite’s Refractive Index: 1.691 – 1.700

 

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’ll be able to 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: 0.008 – 0.013

 

Single or Double Refraction

Tanzanite is slightly doubly refractive.

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.

Tanzanite is transparent.

 

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.

 

Tanzanite’s Specific Gravity: 3.100 – 3.450

 

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 tanzanite 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.