Identify Calcite

Calcite is fairly common and easy to identify when it is in crystal form but don’t worry I will cover the most common types and how to quickly ID them. The reason it is so common, it’s found worldwide in metamorphic, sedimentary, and igneous rocks.

I’ll be focusing on the higher quality material that is dug out of the ground and collected by rockhounds but don’t forget Calcite is used as an abrasive, construction material, agricultural soil treatments, pigments, and more. 

 

How to Identify Calcite 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 Calcite like a pro. 

 

 

Visual Inspection

The visual inspection starts with what form of Calcite 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 some pitting and you’ll see cracking that breaks through the surface, this is normal. It is fairly common to see Calcite cut en cabochon which helps display the banding and its color.

 

Is it faceted? Due to its low hardness on the Moh’s scale, Calcite doesn’t make great material for faceting. Can you facet Calcite? Yes, you can but there’s no demand for it in a commercial setting. Plus, it won’t hold up well in a ring.

 

 

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

 

• Calcite crystals can be opaque to transparent.
• Some Calcite specimens fluoresce under ultraviolet light.
• Can be triboluminescent.
• Luster ranges from vitreous to dull.

 

 

Is it tumbled? It is very common to find tumbled Calcite. It will have a medium polish and is often found in banded varieties.

 

 

Physical Properties of Calcite

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

 

Color: Colorless, White, Red, Yellow, Orange, Blue, Green, Brown, Purple, Black, Multicolored, Banded 

Clarity / Transparency: Transparent, Translucent

Luster: Vitreous, Sub-Vitreous, Waxy, Resinous, Pearly

Cleavage: Perfect {1011}

Fracture: Conchoidal

 

The Streak Test

This is a destructive test, so you must ensure that you can damage the specimen or stone if you choose to use this method.  You won’t use destructive tests once you’ve developed robust knowledge in identifying rocks and minerals.

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

Calcite is not naturally attracted to magnets. However, if metallic coloring agents have been used to enhance a specimen, they will occasionally show a slight attraction.

 

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.  Calcite has a hardness of 3 on the Mohs hardness scale.

 

 

Refractive Index Test

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

 

Calcite’s Refractive Index: 1.658 – 1.486

 

Each gemstone has its own RI, so discovering a sample’s RI can help you figure out what sort of stone it 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: 0.172 (max) 

 

 

Single or Double Refraction

Calcite displays 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, 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.

Calcite 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.

 

Calcite’s Specific Gravity: 2.7102

 

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 hefty 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 Calcite 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.