Rubies have been mined for centuries because of their bright red color and rarity. Most casual owners of rubies are not aware sapphires and rubies are the same mineral and the color is what differentiates them.
Corundum is the mineral class ruby is a part of and all colors, besides blue and red, are known as fancy-colored sapphires.
Aside from its bright color, it’s desirable due to its hardness, luster, durability, and rarity. Rubies are the red variety of a mineral called corundum, with sapphire being another gem variety.
In essence, rubies are red sapphires, and because of this, they are identical in all properties aside from color.
How to Identify Ruby 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 ruby like a pro.
The visual inspection starts with what form of ruby 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? When dealing with a cabochon, it should have a medium to high polish with very little pitting on the surface. Unlike faceted rubies, cabochon rubies will have a smoothly polished, dome-shaped surface and appearance. They’re generally found in oval and round shapes and display a 6-ray star, known as asterism. I went a step further and put together a “how to identify star rubies” post for your review.
Is it faceted? Most commercially available rubies are faceted but the quality will vary dramatically. You’ll need a 10x loop to view the interior of the stone because you’ll be using the internal inclusions to help confirm the gemstone’s identity. Rubies will have “silk” like inclusions and color banding. If the stone is missing these then it could mean you’re looking at a synthetic ruby or a different gemstone altogether.
Is it a specimen? Ruby 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 Ruby displays when it’s a specimen.
- Distinctive red hue.
- Raw Rubies prefer to grow in flat, hexagon shapes. If the uncut gemstone shows this sort of growth pattern and matches the other characteristics we’ve discussed, it’s likely a Ruby.
- Fingerprint inclusions are typically present. Fingerprints are common in rubies because they’re evidence of fractures occurring and healing naturally in the stone. Many other stones cannot naturally heal fractures.
- Color banding. You’ll see this in cabochons where there’s a line of red and then a line of silver/white. These are growth bands.
Is it tumbled? Very rarely do you see rubies in a tumbled form. Even low-quality cab-grade ruby is not typically tumbled because of its hardness.
Physical Properties of Ruby
Let’s take a look at the physical properties of ruby. Knowing what to look for will help you more easily identify what you’re looking at.
Color: Bright Red to Dark Reddish-Brown
Clarity / Transparency: Transparent to Opaque
Luster: Vitreous to Adamintine
Cleavage: None (but may exhibit parting)
Fracture: Conchoidal, Splintery
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.
Tumbled specimens are tested by scraping a specimen across a piece of ungalvanized porcelain, typically known as a streak plate.
Ruby is not magnetic, so it shouldn’t respond to ordinary magnets. However, there are exceptions to the magnet test.
For instance, it’s entirely normal for stones like Hematite to contain a decent amount of magnetite. Since iron ore minerals are formed similarly, the two types may have combined during their chemical formation process. When this occurs, the chemical composition of Hematite changes, making it weakly attracted to magnets.
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, Ruby has a hardness of 9 on the Mohs hardness scale.
Refractive Index Test
Determining the refractive index, or RI as it’s referred to by gemologists, for Ruby 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.
Ruby’s Refractive Index: 1.76 – 1.78
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 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.
Birefringence: 0.005 – 0.009
Single or Double Refraction
Rubies have a double refraction, meaning when light enters them, it splits into beams.
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, 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 remains light, it is likely AGG. If the stone’s lightness or darkness changes, DR is likely.
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.
Rubies are transparent to opaque. However, its transparency depends on the form it has taken. If the ruby has a solid crystal form with earthy skin then 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.
Ruby’s Specific Gravity: 4
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 ruby 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.