New gemology students often fuss over inconsequential data when identifying gems for the first time. In the process, they miss important clues. Getting practical experience is vital. In order to help make the transition from the textbook to the real world, I’ve created a gem identification quiz series.
By Donald Clark, CSM IMG 4 minute read

“What is it?” Quizzes

gem identification quiz - green gem

Peridot, emerald, or something else? This quiz can help gemology students learn how to choose identification tests and run them efficiently.

These quizzes are designed to be as realistic as possible. Hopefully, they’ll give student gemologists a better idea of how to proceed based on the information at hand. In this manner, they’ll learn not only the process but also time-saving shortcuts.

You’ll find the first gem identification quiz below. The rest of the series is available to those who become members of the International Gem Society (IGS).

Gem Identification Quiz #1: A Green Stone

A customer brings you a green, transparent stone (shown on the right) and wants to know if it’s an emerald or a peridot. Peridot is her birthstone, but she know emerald is much more valuable! With fingers crossed, you begin your examination.

Loupe Exam

First, clean the gem and examine it with a 10X loupe. You find a small feather inclusion and note the stone is very well-cut, but nothing more.

Microscope Exam

Next, take the stone to your microscope and examine it thoroughly with darkfield and oblique illumination. You see it has a small conchoidal fracture with vitreous luster on the culet. However, you don’t find any additional inclusions.

Refractometer Exam

Use your refractometer to take a basic refractive index (RI) reading. (Two readings at 90° to each other). You note your readings: 1.621 and 1.627.

Polariscope Exam

A quick check with a polariscope shows that the stone is doubly refractive. However, you can’t find an optic figure and sign. These would indicate its optic character. While examining the stone, you notice that it’s at least moderately pleochroic with yellowish green/green colors

Review What You’ve Found

I chose these initial tests because they’re the easiest and least time-consuming. Look at what you learned about your gem in just a few minutes: the approximate RI range, double refraction, and moderate pleochroism.

Of the other clues you’ve found, the feather and the conchoidal fracture with vitreous luster are of least importance. Unfortunately, these are so common they have little identification value.

You also know the stone is green and transparent. Yes, dozens of stones possess these qualities. Nevertheless, they still limit the possibilities a bit. For example, you can eliminate malachite, turquoise, and opal.

Usually, being well-cut indicates a valuable gem. Few people would spend much time on a synthetic. However, this proves nothing. Just keep this clue in mind. It may be important later.

Take the Gem Identification Quiz

After these initial tests, consult your reference materials and try to narrow down the options for your gem’s identity. Now, answer the following questions:

Question 1

At this point, what do you think the stone is, most likely?

Question 2

What test or tests would you run next?

Gem Identification Quiz Answers

Question 1

Take the information you’ve collected and consult gemological references. So far, you have the following data:

  • Color: green.
  • Optic Character: doubly refractive, but could find no figure.
  • RI high and low: 1.621 and 1.627. (Note: use what you measured, but the actual range may be greater).
  • Pleochroism: 2 colors and moderate.
  • Transparency: transparent.

The possible IDs you’ll find will most likely include topazactinolite, and a few varieties of tourmaline. You don’t need to identify the tourmaline variety at this point. Therefore, you need a test or tests to distinguish topaz, actinolite, and tourmaline.

Question 2

Topaz has a significantly different specific gravity (SG) than the other possibilities, but SG won’t distinguish actinolite from tourmaline. However, each of these gems has a different optic character: topaz is biaxial positive, actinolite is biaxial negative, and tourmaline is uniaxial negative. So, that’s the deciding factor. Since you couldn’t find the optic figure with your polariscope, try using your refractometer to determine its optic character. When you’ve done that, you can make a positive identification.

Quiz Follow-Up

Hopefully, this little lesson helps you understand the gem identification process. Gather your initial data, review your findings, consult your references, eliminate possibilities, and choose additional tests.

So, what is that green gem? That depends on its optic character. I haven’t revealed the answer. The quiz is designed to help you learn to winnow data and pick appropriate tests.

However, here’s a little more information about one of the possibilities.

Typically, you can easily identify tourmalines. Strong pleochroism is a good indicator, especially if you see two entirely different colors, like orange and green. Another quick way to identify tourmalines is finding hollow growth tubes. Either of these qualities indicate you’re likely looking at a tourmaline. Next, check the RI. If it matches tourmaline, the only other thing you need for proof of identity is the optic character. No other gem has strong pleochroism, the same RI, and optic character as tourmaline.

Tourmalines frequently have stress, which should make finding the optic figure in the polariscope easy. However, even if you don’t find it with that tool, you can still use your refractometer.

If you’re an IGS member, take a look at the entire quiz series.