Kyanite is an aluminum silicate mineral that is well-known by rockhounds. What makes this mineral unique is its variable hardness and perfect cleavage. When cut parallel (lengthwise), it has a hardness of 4 to 4.5, but when the same stone is cut perpendicular, its hardness is 6 to 7.5.
High-end specimens are sought after by collectors while the brave and daring lapidary artist will try to facet or cab rough kyanite. You’re probably asking yourself why would someone try to facet a stone with two different hardnesses. If you get the chance to see a deep blue kyanite crystal cut into a faceted oval or emerald cut then you’ll understand why the artist took the risk because they’re amazing to look at it.
How to Identify Kyanite 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 kyanite like a pro.
The visual inspection starts with what form of kyanite 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? If you’re dealing with a cabochon, it should have a medium polish with layers of cracks coming to the surface. Cabochon kyanite will contain inclusions and range in color from clear to shades of blue to green and black.
Is it faceted? If you have a faceted piece of kyanite, it should have quite a few visible inclusions and the carat weight should be in the 2-7 range.
Is it a specimen? Kyanite is found in different forms, and you’ll get better at identifying these forms by looking at and inspecting this mineral over time. Here’s a list of characteristics kyanite displays when it’s a specimen.
- Blue to blue-gray to barely bluish-gray bladed crystals.
- You’ll be able to do the streak test. Keep reading below if you have a rough piece with no commercial value.
- Rough crystalline structure.
- Long slender specimens.
Is it tumbled? Very common to find tumbled Kyanite. Inclusions will be present with cracks across the surface of the stone and the polish will be medium while the colors will range from white, light gray, and various shades of blue.
Physical Properties of Kyanite
Let’s take a look at the physical properties of kyanite. Knowing what to look for will help you more easily identify what you’re looking at.
Color: Colorless, White, Light Gray, Blue, Green, Orange, Pink, Yellow (Rarely)
Clarity / Transparency: Transparent, Translucent
Luster: Sub–Vitreous, Vitreous, Pearly, Greasy
Cleavage: Perfect on 100, Good on 010
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 Kyanite, you’ll notice a white streak.
Tumbled specimens are tested by scraping a specimen across a piece of ungalvanized porcelain, typically known as a streak plate.
Kyanite is not magnetic, so it shouldn’t respond to common magnets. However, there are exceptions to the magnet test.
For instance, when Kyanite contains traces of chromium and iron, it can exhibit magnetic properties. However, because not all samples and specimens contain these minerals, the magnet test wouldn’t be a surefire way to identify the stone. That said, your specimen may or may not exhibit magnetic properties.
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, Kyanite has a hardness of 4.5 to 5 along the length of the crystal and 6.5 to 7 across the width on the Mohs hardness scale.
Refractive Index Test
Determining the refractive index, or RI as it’s referred to by gemologists, for Kyanite is reasonably 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.
Kyanite’s Refractive Index: nα = 1.712 – 1.718 nβ = 1.720 – 1.725 nγ = 1.727 – 1.734
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.
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: δ = 0.015
Single or Double Refraction
Kyanite has a double refraction.
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.
Kyanite is transparent to translucent. However, its translucency depends on the form it has taken. If the Kyanite 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 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.
Kyanite’s Specific Gravity: 3.5 – 3.7
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, 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 kyanite 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 get confused, feel free to reach out, and I’ll do my best to assist you in the identification process.