Gemologist Guide to Identifying Prasiolite

Prasiolite is very common and you’ll see it at every rock and mineral show you attend. Most of the time, prasiolite has been cabbed and faceted into different shapes and sizes which are used in jewelry and beaded bracelets.

Due to it being fairly inexpensive, prasiolite is worn and collected by most rockhounds and crystal enthusiasts.

 

 

How to Identify Prasiolite 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 prasiolite like a pro.

 

Visual Inspection

The visual inspection starts with what form of prasiolite you have.  The questions below are fairly easy to answer, but each type will have its own process for identifying them. The first thing we need to address is the fact that prasiolite is actually amethyst. Gem dealers take light-colored amethyst and heat or irradiate it which turns it green.

 

 

 

Is it a cabochon? If you’re dealing with a cabochon, then it should have a medium to high polish with very little pitting on the surface. The inclusions create a beautiful array of whitish and soft grey hues. You’ll also find higher-quality pieces with no inclusions used in rings and pendants.

While prasiolite cabochons are popular in jewelry, it’s rarely seen in high-end fashion. Beaded bracelets and necklaces are fairly common and crystal collectors will use them during affirmations.

 

 

 

Is it faceted? If you have a faceted piece of prasiolite, then it should be transparent with colors ranging from light green to medium green. As you can see from the high-quality faceted prasiolite above, medium saturation with a medium green color is the most sought-after.

When inspecting the stone you’ll want to use a 10x powered loop to scan for internal inclusions. Prasiolite will have feathers and clouds but most stones will have a high clarity rating. Keep in mind green beryl looks similar to prasiolite but it lacks the grey undertones when it comes to color.

If you need help valuing and pricing prasiolite then use my free guide.

 

 

 

Is it a specimen? Prasiolite 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 prasiolite displays when it’s a specimen.

• Pale to medium-colored green
• Translucent – transparent
• Large chunks are common

 

Is it tumbled? Very common to find tumbled prasiolite. It will have a smooth, glossy shine.

 

Physical Properties of Prasiolite

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

 

Color: Pale green to medium green

Clarity / Transparency: Transparent to Translucent

Luster: Vitreous

Cleavage: None

Fracture: Conchoidal

 

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 is when you scrape the stone against a harder surface to see what color remains.  Prasiolite doesn’t leave a streak because it’s harder than the streak plate.

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

 

Magnet Test

Prasiolite is diamagnetic, so it shouldn’t be attracted to common magnets.

Instead of an attraction, prasiolite is forced out of the magnetic field, but the force won’t be strong at all.

 

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.  With that being said, prasiolite has a hardness of 7 on the Mohs hardness scale.

 

Refractive Index Test

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

 

Prasiolite’s Refractive Index: 1.54 – 1.55

 

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 of the gauge.

 

Birefringence Test

You won’t be using this test for prasiolite, but I wanted to include this test just in case you were considering it in your process.

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 = 0.009

 

Single or Double Refraction

Prasiolite displays a double refraction.

or 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 stays 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.

Prasiolite is translucent. However, its translucency depends on the form it has taken. If the prasiolite 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.

 

Prasiolite’s Specific Gravity: 2.6 – 2.7

 

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, then 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 prasiolite 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, then feel free to reach out, and I’ll do my best to assist you in the identification process.