Gemologist Guide to Identifying a Star Ruby

Star rubies are well known and sought after outside of the U.S. market but for some reason, American consumers do not have the same appreciation. If you need help identifying a ruby then go here first.

I wanted to provide additional information about identifying star rubies with their own article. Keep in mind star rubies can only be red or a pinkish red. If you’re dealing with another color then go to the star sapphire identification page.

 

How to Identify Ruby 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 gemstones, then feel free to share it with us.

Let’s take a deeper look into how to identify ruby like a pro.

 

 

Visual Inspection

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? Every star ruby you come across will be in the form of a cabochon or crystal with a polished end. All cabochons will have a high polish with very little pitting but you will see banding inside the stone. These bans will run parallel to each other and occasionally have a 45-degree kink in them.

Not all star rubies will have a perfect 6-ray star. As you can see from the image below, some star rubies will have more of a blotch or circle-shaped star that floats around the stone.

 

 

Is it faceted? Star rubies are never faceted. That doesn’t mean someone can’t cut one but it would be very rare to come across it.

 

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.
• It can be found in crystal form, and these can be quite common. They will be a pinkish red in color and the ends of the crystals will show a sheen when direct light is applied. Your eyes are picking up the asterism in the stone. The star effect is obstructed by the outside of the crystal but when you polish the end, the star will appear.
• 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 uncommon due to its hardness and you’ll want to orientate the stone to get the best star effect. Tumbling would remove this option.

 

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.

 

Magnet Test

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.

 

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.  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.

 

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: 0.005 – 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.