Turquoise is well known in the jewelry industry and rock hounds enjoy digging for it and collecting it. It’s known for its incredible shades of blue and bluish-green and has been a treasured gemstone for centuries. Very few minerals have such a profound and well-known color among professional collectors and the general public.
High-quality turquoise can be difficult to come by while low to medium is generally available at your local gem and mineral show as well as online. If you’re new to buying gemstones then you need to be aware of online sellers trying to sell fake turquoise to unsuspecting buyers. You can use my free guide on how to spot fake turquoise to educate yourself.
How to Identify Turquoise 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 turquoise like a pro.
The visual inspection starts with what form of turquoise 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 polish with pitting and streaks of matrix. Black, gold, and earthy-colored inclusions are beautiful.
Is it faceted? Faceted turquoise is extremely rare because it’s opaque and it doesn’t make sense to facet it. Can you come across a faceted piece of turquoise? Yes, you can but it is not very likely.
Is it a specimen? Turquoise 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 turquoise displays when it’s a specimen.
- It’s common to find specimens with black and gold streaking.
- Specimens are typically small, ranging from quarter-sized and up.
- Some specimens look like brown rocks with Turquoise throughout.
Is it tumbled? Very common to find tumbled turquoise pieces and free-form pieces. Again, there will be a medium to high polish which is dependent upon the quality of the material and where it was mined.
Physical Properties of Turquoise
Let’s take a look at the physical properties of turquoise. Knowing what to look for will help you more easily identify what you’re looking at.
Color: Sky Blue, Blue, Bluish-Green, Green, Yellowish-Green
Clarity / Transparency: Transparent
Cleavage: 2,1 – 3,1
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. Turquoise has a bluish-white to greenish-white streak.
Tumbled specimens are tested by scraping a specimen across a piece of ungalvanized porcelain, typically known as a streak plate.
Natural Turquoise that is blue and green is weakly to strongly magnetic. This is because of the copper phosphate or copper salts. Imitations made from naturally dyed material are easily detected by their inert responses.
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, Turquoise has a hardness of 5 to 6 on the Mohs hardness scale.
Refractive Index Test
Determining the refractive index, or RI as it’s referred to by gemologists, for Turquoise 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.
Turquoise’s Refractive Index: 1.61 to 1.65
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.
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 = 0.040
Single or Double Refraction
Turquoise has a double refraction.
The stone must be transparent for this test to be accurate and beneficial. 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 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.
Turquoise is 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.
Turquoise’s Specific Gravity: 2.5 to 2.9
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. This is the one gemologists use OHAUS Density Determination Kit.
Identifying Rocks and Minerals Like a Pro
Hopefully, you feel confident in your practice of identifying a piece of Turquoise 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.