Step 3: Practical Gemology

Lesson 14

A gemologist can conduct many different tests with a polariscope. Gemstone twinning, dichroism, and internal crystal strain are some of the physical and optical characteristics that can be studied through this instrument.

Gemstone Twinning

A crystal doesn’t always form in a single step. If one of the required elements for a crystal’s growth is depleted, it will stop growing. When the proper mix of ingredients is present again, growth will resume. Some crystals will continue to grow as a whole, without any division. Others will grow in layers, one on top of the other. This is known as twinning.

Twinning is most common in corundum and quartz. It appears in your polariscope as parallel layers of color. Since the layers can be polarized in opposite directions, this can affect your readings. Corundum with extreme twinning will be light in all directions in the polariscope. This usually indicates an aggregate. Check for double refraction with a dichroscope.

Twinning is absent in most synthetic quartz. Some new synthetics show twinning but are rare. If you find twinning in a quartz family gem, it’s probably natural. (Its absence proves nothing).…