Step 3: Practical Gemology

Lesson 28

Gemstone Examination: Example 1

This stone looks like a tourmaline.

I made that initial assessment based partly on its shape (tourmaline crystals are elongated, so rectangular gems are common) and partly on its color (green, but slightly grayish). It’s certainly not an emerald or other chromium-colored gem.

With the loupe, I saw that it’s well-cut. It had some fingerprints and fractures, but no identifying inclusions.

The end facets are black. No light was passing through them. This is called a “closed C axis” and is common to tourmaline, but little else. For our purposes, we’ll simply note that the stone has strong pleochroism.

Next I took a basic refractive index (RI) reading. On the long axis it measured 1.643. Turning it sideways, it read 1.641. I put the stone in the polariscope, which verified that it was doubly refractive. It didn’t show any stress. Since I thought I had enough information to prove it was tourmaline, I didn’t make much effort to find the optic figure and sign.

Reviewing the References

I reviewed my gemological references for stones with the following characteristics:

  • Color: green