Gemstone Identification Tests: How I Identify Gemstones
Identifying a gemstone is a process of elimination.
Gemologists gather visual clues and use them to eliminate stone types until only one remains, the identity of the gem.
Within this process, we don’t have to carry out every test for every gemstone. Knowing what test to do next is the trick to identifying gemstones efficiently.
Personally, I always start with a few quick tests to gather key details:
Colour
Transparency
Whether it's singly or doubly refractive
Pleochroism (and if so what are the 2/3 colours)
Refractive index (RI) reading(s)
Reaction to long-wave UV light
I then use a handy computer program to generate a shortlist of possible gemstones, which helps me decide what to test next.
Think of it like the game Guess Who? the goal is to eliminate as many options as possible with the fewest number of clues, so you can reach the right answer quickly and efficiently
(Side note: When I first started, I used a chart of RI readings and manually looked up each gem with matching RI and birefringence. It worked, but it was slow. These days, I use a brilliant program called Gemology Tools Professional which I highly recommend if you're into gem testing.)
Gemstone knowledge is another key part of this process. While you can’t confirm a gemstone’s identity without testing it, knowing what it might be can help you choose the most useful tests and speed up the process even more.
So, what are the characteristics gemologists test for, and how do we test them? Let’s break it down.
Optical Character: Singly vs. Doubly Refractive
This tells you whether light entering the gem travels in a single path or splits into two directions.
Most (but not all) gemstones are doubly refractive, while glass is singly refractive. When I have a big pile of “gemstones” to test, I run each one through my polariscope to check its optical character. This lets me separate them into three piles:
DR - 'Definitely some kind of gem'
SR - 'Could be glass'
AGG - 'Aggregate gems'
I don’t just do this for motivational reasons (though yes, it is more fun to identify a gemstone than a bit of glass!). It’s also useful information.
Singly refractive stones won’t be pleochroic, and they’ll only show one RI reading, so it’s good to know this before carrying out any more tests.
📍 How to test it:
(many of the properties we test can be obtained in multiple ways)
Polariscope
Quick and easy! You rotate the gem between two polarised filters.
A DR (doubly refractive) gem will blink light and dark
A SR (singly refractive) gem stays constant dark
An aggregate gem stays light all the way round
A fourth possible result is a swirling snake-like pattern, which usually indicates single refraction. But not always. I typically include these in my ‘could be glass’ pile, or you can make a separate group for uncertain results if you'd rather.
Refractometer
If the gem is DR, the refractometer will show two RI readings. It does save on running two separate tests since you'll want to get the RI reading(s) anyway, but personally I prefer to know what I'm looking for before I get out my refractometer.
Refractive Index (RI)
RI is one of the most useful and reliable properties we can test. Personally I never identify a gemstone without having checked it.
📍 How to test it:
Refractometer
This is the most accurate tool for testing a gem’s refractive index. You apply a drop of RI fluid and place the gem table-down on the glass plate (you can use any facet, but the table is the biggest so try that one first). The reading can be a single value (for singly refractive stones) or two values (for doubly refractive ones).
Submersion Test
(Not recommended)
If you don’t have a refractometer, you could get a crude RI estimate by submerging the gemstone in a liquid with a known RI. The level of visibility tells you if the stone has a higher or lower RI than the liquid. This isn’t a reliable identification method, and the liquids with higher RI's tend to be highly toxic, but as it is technically a means of getting a rough RI readings I've included it here.
Optic Sign
Tells us if a DR gem is uniaxial (+ or -) or biaxial (+ or -), a key ID marker for many stones.
📍 How to test it:
Polariscope + Conoscope
When you orientate a gemstone within the polariscope so you're looking through it's optical axis you will see a rainbow like effect on the surface of the gemstone, kind of like the colourful sheen you get when oil is floating on water.
You can then use your conoscope to see an interference figure. The different patterns these figures take tell the gems optical sign (and sometimes they'll even tell you that the gem is a type of quartz). This method is quick, but can't always be done as you won't always get a clear interference figure.
Refractometer
More reliable than a conoscope for difficult stones, but much slower. You’ll need to take multiple accurate readings and calculate the optic sign manually based on the movement of the RI readings.
Birefringence
Birefringence is the difference between the highest and lowest RI in a doubly refractive gemstone.
📍 How to test it:
Refractometer
Simply subtract the lowest RI from the highest to get the birefringence value.
(If you're using the software I mentioned above, it does this for you automatically when you enter the RI figures)
Pleochroism
Pleochroism is when a gem shows different colours from different directions, which is common in many DR gems.
📍 How to test it:
Dichroscope
A pocket tool that shows two pleochroic colours when the gem is viewed through it. It's worth turning the gemstone under the dichroicope to see if different angles produces a third colour. Easy, fast, and very useful for gem IDing.
Naked eye observation
Some pleochroism is visible without tools, especially in stones like tanzanite or andalusite. Rotate the stone and look for colour changes across different axes.
Fluorescence
Some gemstones glow under UV light. This can help distinguish between similar-looking stones or flag synthetics and treatments.
📍 How to test it:
Shortwave and Longwave UV Lamps
Observe the gem’s reaction under UV light. Ruby, for example, often fluoresces strongly red under longwave UV, unless it's synthetic or heavily treated. Some diamonds glow blue and some synthetic spinels fluoresce orange or red.
Inclusions and Internal Features
Inclusions can be diagnostic. They help us identify natural vs. synthetic and may hint at geographic origin or treatment history.
📍 How to test it:
10x Loupe and/or Microscope
A gemologist’s best friend. Look for telltale inclusions like rutile needles (silk in sapphire), curved striae (in flame-fusion synthetics), or flux inclusions (in synthetic emeralds). Surface features like chips and polish lines can also tell you a lot, including if the "gemstone" is actually molded glass or plastic instead of a cut stone.
Treatment Detection
Spotting treatments is a key part of gem testing, especially for high-value stones like sapphires and rubies.
📍 How to test it:
Microscopic Examination
Look for colour concentrations in unusual places, strange flashes of colour within the gemstone, surface pits, or inclusions that have melted or altered. These can all indicate heat treatment, filling, or diffusion.
You can also check a gemstone more easily for colour coating using a microscope as the gems surface will often look mottled. I find is especially noticeable on colour coated topaz.
Immersion (Submersion) Testing
Placing a gemstone into a liquid with a similar refractive index (RI) makes it easier to see internal features. For a simple example of this test in action, try placing a piece of quartz in water. It becomes nearly invisible, almost like a little jelly blob.
This method of immersing a stone can help reveal treatments or inclusions that might otherwise be hard to detect.
For example, diffusion-treated sapphires often show colour concentrated near the surface, while the centre of the gem may be noticeably lighter, or even an entirely different colour. Immersion testing can also reveal filled fractures, as the filling usually has a different RI than the host gem, making the treatment more visible under immersion.
Side note: As I mentioned earlier, liquids with higher RIs (like those close to corundum) are often quite toxic. Personally, I use cinnamon bark oil. It doesn’t have as high an RI as the more conventional heavy liquids used by other gemmologists, and it still requires careful handling, but as a former aromatherapist, I’m far more comfortable using an essential oil than a hazardous chemical
Spectroscope Readings
Some gems absorb light in distinctive patterns. These absorption lines or bands can help confirm identity.
📍 How to test it:
Handheld Spectroscope
Shine a bright light through the gemstone and look for characteristic lines. For example, chromium-rich stones like ruby show distinct absorption in the red and blue-violet areas.
Specific Gravity (SG)
SG tells us how dense a gem is compared to water. It’s less commonly used for faceted stones as it becomes less accurate the smaller the stone is, but it can still be useful when other tests are inconclusive.
📍 How to test it:
Hydrostatic Weighing
Weigh the stone in air and again in water (you need to suspend the gem in the water without it touching the sides or the bottom). The difference between the two readings is the gems SG. You can buy scales made specifically for this type of testing, or, if you want a cheaper option, you can make your own if you're good at soldering.
Heavy Liquid Testing
Drop the stone into a liquid of known SG and see if it floats or sinks. This can give a rough idea but requires care and safety precautions.
Gemstone identification is both a science and an art. It takes practice to know which tests to run and when, but once you’ve built up your skills and intuition, the process becomes faster and more rewarding.
If you're thinking about testing gemstones at home, a basic gemology kit and a good reference program like Gemology Tools Professional will take you far. And if you're ever in doubt about your results, test again. Don’t just assume. Even the prettiest stone can surprise you!