Gemologist Guide to Identifying Iolite

Iolite is the gemstone variety of cordierite and displays a light to deep blue color and occasionally you’ll notice a purple tinge.  The more saturated stones demand a premium price but you run into clarity issues with this gemstone. Most material is heavily included which makes a high-clarity stone more valuable. The stones exhibiting a light to medium blue color are fairly inexpensive and can be found in jewelry.

If you can’t afford a sapphire or tanzanite then iolite is a great substitute.

When viewing a faceted or cabbed piece of iolite you’ll notice it exhibits a strong pleochroic property which means you see a different color depending on which angle you view the stone from.

Fun fact: Iolite is one of the most pleochroic stones, with the effect being noticeable on all transparent varieties. When viewed at different angles, blue or blue-violet iolite will become duller gray or grayish-yellow. Because of this, lapidaries must orient the stone correctly before they start cutting the stone.

 

How to Identify Iolite 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 iolite like a pro.

 

 

Visual Inspection

The visual inspection starts with what form of iolite 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 medium to high polish with little to no pitting. If you see pitting then it is the internal inclusion breaking the surface of the cabochon. Most of the material will be transparent with grey-looking inclusions. To quickly identify this gemstone you’ll want to see the pleochroic properties from the surface.

 

 

Is it faceted? You want to look for the pleochroic properties and the color should line up with iolite. Break out your 10x powered loop to view the internal inclusions and you’ll notice feathers and clouds which will have a grey color. When viewing the stone you’ll notice different colors down the A axis and the Y axis.

 

 

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

• Specimens are often semi-transparent to transparent.
• Deep blue-violet colors are common.
• When turned 90 degrees, some areas can be almost colorless to a pale brown.
• Pleochroism will be easily seen.

 

 

Is it tumbled? Very common to find tumbled iolite. It will have a medium to high polish. Tumbled Iolite is usually quite dark but still shows varying shades of violet, navy, or steel blue.

 

Physical Properties of Iolite

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

 

Color: Blue, Gray, Slight Violet Blue, Near Colorless

Clarity / Transparency: Translucent to Transparent

Luster: Vitreous

Cleavage: Distinct in One Direction

Fracture: Conchoidal to 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. Iolite has a colorless streak. This is because a streak plate’s hardness is only 6.5, so minerals harder than the plate will not leave a streak.

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

 

Magnet Test

Iolite is colored by iron, which contributes to the stone’s strong magnetic attraction to an N52 magnet when floated.

 

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.  Iolite has a hardness of 7 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 iolite, 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.

 

Iolite’s Refractive Index: 1.522 to 1.578

 

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: 0.005 to 0.018

 

Single or Double Refraction

Iolite has a weak to medium 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.

Iolite is transparent to translucent.

 

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.

 

Iolite’s Specific Gravity: 2.53 to 2.78

 

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 iolite 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.