Lapis Lazuli, commonly known as “Lapis,” is a beautiful blue metamorphic rock that has been used as a gemstone, sculpting material, ornamental material, and pigment for thousands of years. The most sought-after specimens have a rich, solid blue color with a few reflective pieces of gold pyrite. Unlike many other gem materials, lapis isn’t a mineral. Instead, it’s a rock composed of multiple minerals. The shades of blue are primarily derived from the presence of lazurite.
How to Identify Lapis Lazuli 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 lapis lazuli like a pro.
The visual inspection starts with what form of lapis lazuli 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 little to no pitting. Lapis Lazuli cut en cabochon will contain gold and white veining which resemble streaks. Some cabbed lapis will have black colored streaks instead of white ones. As you can see in the image above, most cabochons will be in standard shapes and sizes but some lapidary artists will cut them in free-form shapes.
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Is it faceted? I’ve been cutting stones for 20+ years and I have never come across a faceted piece of lapis. That doesn’t mean someone couldn’t facet one but it would be extremely rare.
Is it a specimen? Lapis Lazuli 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 lapis lazuli displays when it’s a specimen.
- Ranges from deep, dark blue to lighter shades of blue.
- Golden pyrite and white streaks will be present.
- Streaks of black can be found in the material
Is it tumbled? Very common to find tumbled lapis lazuli ranging in different sizes and shapes. You will rarely find high-quality lapis in tumbled form but low-quality is tumbled on a regular basis. When looking at the polish on the stone you will see a low to medium polish.
Physical Properties of Lapis Lazuli
Let’s take a look at the physical properties of lapis lazuli. Knowing what to look for will help you more easily identify what you’re looking at.
Color: Blue – Often contains white calcite veining or mottling and gold grains of pyrite
Clarity / Transparency: Semitranslucent to Opaque
Luster: Vitreous, Waxy
Fracture: Granular, 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. Lapis Lazuli always produces a light blue streak.
Tumbled specimens are tested by scraping samples across a piece of ungalvanized porcelain, typically known as a streak plate.
Lapis Lazuli’s color includes sulfur. It’s composed of diamagnetic minerals like Calcite and Lazurite, along with some flecks of pyrite. Pyrite is a weakly magnetic iron sulfide. That said, lapis lazuli should show a magnetic attraction unless, by chance, it contains a high amount of pyrite. Even so, because pyrite is weakly magnetic, any pull would be very weak.
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, lapis lazuli has a hardness of 5 to 5.5 on the Mohs hardness scale.
Refractive Index Test
Determining the refractive index, or RI as it’s referred to by gemologists, for lapis lazuli 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.
Lapis Lazuli’s Refractive Index: 1.500 to 1.522
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.
Single or Double Refraction
Lapis Lazuli is singly 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.
Lapis Lazuli is semitranslucent to 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.
Lapis Lazuli’s Specific Gravity: 2.500 to 3.000 Typical: 2.750
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 lapis lazuli 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.