Identify Cuprite

Cuprite is such an interesting stone and fairly easy to identify because it has a couple of unique characteristics. I’ll get into the details below but if you have a chance to collect or purchase Cuprite then I highly recommend you do.

Occasionally, specimen dealers call Cuprite, red copper ore. It’s a soft, yet heavy, red oxide mineral that’s a major ore of copper. 

Cuprite crystals are transparent to translucent and have a metallic-looking sheen but when you put a light behind it, magic happens. The crystal or specimen will glow an intense red but when you take the light away the stone appears to be opaque.

 

How to Identify Cuprite 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 Cuprite like a pro. 

 

 

Visual Inspection

The visual inspection starts with what form of Cuprite 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? Cuprite is fairly soft and doesn’t make good cabochon material. I highly doubt you will come across a cabbed piece of Cuprite.

You should consider other metallic-looking minerals before you settle on Cuprite.

 

Is it faceted? If you have a faceted piece of Cuprite, it should appear to be opaque but you might see a red flash. The minute you shine a light on the stone or put the light behind the stone, it should glow red. 

When looking at the faceted stone you want to see a metallic finish, red flash. Then see if it glows red with a light. If it meets this criteria then you probably have Cuprite but you might have Rutile. 

 

 

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

• Cuprite crystals appear as cubic, octahedral, or dodecahedral forms. It can also form in a combination.
• It’s widespread as crystals, grains, or earthy masses.
• Often found in a copper matrix.

 

Is it tumbled? I don’t believe you will come across tumbled Cuprite, too soft.

 

Physical Properties of Cuprite

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

 

Color: Dark Red to Cochineal Red (sometimes almost Black) 

Clarity / Transparency: Transparent, Translucent

Luster: Adamantine, Sub-Metallic, Earthy

Cleavage: Imperfect/Fair Interrupted on {111} 

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. Cuprite produces a shining metallic brownish-red streak.

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

 

Magnet Test

Since Cuprite is a major ore of copper, it’s diamagnetic. Instead of being attracted to magnets, the stones are repelled by them.

 

Hardness Test

I don’t recommend actively testing the hardness of a stone because it’s destructive in nature and doesn’t provide a definite answer to what type of stone it is.  Cuprite has a hardness of 3.5 to 4 on the Mohs hardness scale.

 

Refractive Index Test

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

 

Cuprite’s Refractive Index: 2.849

 

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: None – Isotropic minerals have no birefringence.

 

 

Single or Double Refraction

Cuprite displays a single 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.

Cuprite 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.

 

Cuprite’s Specific Gravity: 6.14

 

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 Cuprite 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.