Opals are very common but there are lesser know varieties as well. It is recognized as the October birthstone and most of the material is sourced from Australia.
There are multiple deposits of opal across the world and each one has a unique look due to the matrix it grows in.
Individual opals can vary significantly in quality and appearance. They’re as diverse and unique as snowflakes and as individual as a snowflake. The three main aspects regarding Opal’s quality are:
- Patterns – The arrangement of play-to-color.
- Color – The background color and play-of-color.
- Clarity – The quantity and transparency of inclusions.
But we’re here to discuss identifying opal, so let’s dig in.
How to Identify Opal 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 feel free to share it with us.
Let’s look deeper into how to identify opal as a pro.
The visual inspection starts with what form of opal 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? If you’re dealing with a cabochon, it should have a high polish and can have pitting. Most of the cabs will be in a free-form style and have a white base, grey base, or translucent appearance. When inspecting the cab you want to view it horizontally to ensure it’s not a doublet or a triplet.
There are synthetic opals but they tend to be less common and are easily spotted. When viewing the synthetic through a 10x powered loop you’ll see columns of color. When viewing natural opal you’ll see colors that flow and move.
Doublets and triplets are created when high-end opal is too thin to be used in jewelry but the cutter doesn’t want to waste the material. So, they create a sandwich of material to make the opal thicker.
Opal Doublet – This is made up of an opal-matrix base and a transparent top (two pieces of material sandwiched together). The top of the cab will be made out of quartz, a synthetic, or glass while the bottom of the cab is actual opal.
Opal Triplet – This is made up of a dark base, typically onyx or inexpensive dark material, a very thin piece of opal, and a transparent top. The top of the cab will be made out of quartz, synthetic, or glass. By using three pieces to form the cabochon we give it the “triplet” name.
Is it faceted? Opal as you know it doesn’t come in a faceted form but if you’re a collector or rockhound then you’re probably aware of Mexican fire opal, Oregon opal, and opalized wood. Cutters are able to facet stones from this rough but it will be on the soft side and doesn’t hold up well in jewelry. Some of the pieces will show fire or color dancing around but most are either yellow, orange, or red in color. It will have very little dispersion and the light reflected back to your eye will be cloudy or soft looking.
Is it a specimen? Opal is found in different forms. You’ll get better at identifying these forms by looking at and inspecting this mineral over time. Here’s a list of characteristics Opal displays when it’s a specimen.
- The base color can be clear, honey yellow, orange, orange, reddish-orange, or white.
- Commonly found in orange and honey colors.
- Clarity varies from transparent, to translucent, to opaque.
Is it tumbled? Very common to find tumbled Opal. Shiny, bright, colorful, and sometimes fluorescent. Many tumbled opal are opaque and white but can be as colorful as a neon sign from the 1980s.
Physical Properties of Opal
Let’s take a look at the physical properties of opal. Knowing what to look for will help you more easily identify what you’re looking at.
Color: Colorless, White, Red, Yellow, Green, Orange, Blue, Black, Brown
Clarity / Transparency: Transparent, Opaque, and rarely Translucent
Luster: Waxy, Vitreous, Dull, Greasy
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. Opal has a white streak.
Some rough specimens can be streaked across a piece of white paper to expose its color.
Tumbled specimens are tested by scraping samples across a piece of ungalvanized porcelain, typically known as a streak plate.
Opal is not magnetic, so it shouldn’t respond to common magnets. However, there are exceptions to the magnet test.
For instance, diamagnetic (can become magnetized in a direction at 180 degrees to a magnet) gems like Opal and Quartz contain macro and microscopic inclusions that can cause magnetic attraction – paramagnetism. When the inclusions fill with minerals like pyrite, which has a weak magnetic attraction, the Opal may weakly respond in return.
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, Opal 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 Opal 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.
Opal’s Refractive Index: nω = 1.400 – 1.460
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.
You won’t be using this test for opal, 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 = None
Single or Double Refraction
You won’t be using this test for opal, but I wanted to include this test just in case you were considering it in your process. For this test to be accurate and beneficial, the stone needs to be transparent in nature. 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 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. When they’re thick, a small amount of distortion might occur, but light will pass through them relatively freely.
Opal is translucent. However, its translucency depends on the form it has taken. If the Opal 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.
Opal’s Specific Gravity: 2.0 – 2.2
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, 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 opal 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, feel free to reach out, and I’ll do my best to assist you in the identification process.