Fall 2009

A Useful Technique to Identify Negative Crystals in Ruby

Although negative crystals in corundum are common, they are often mistakenly assumed to be crystals of a foreign material. By combining a little knowledge of crystalline materials with a useful technique for microscopic examination, we can find clues that reveal their true identity. In particular, since the host mineral’s crystal structure defines the shape of these negative spaces, when several of them are clustered together their identity is often revealed by their congruent arrangement.

Recently, a very good example of this was observed in a ruby submitted to the Carlsbad laboratory for a Ruby Report. To verify the inclusions in question as negative crystals, a simple tool was constructed to induce or block reflections on their internal faces by controlling their exposure to light. A small square (~one inch, or 2.5 cm) of black electrical tape was affixed to a pointer probe and inserted between the well light of the microscope and the ruby. As the tape was moved in and out of the well to partially block the light, a uniform change in reflections throughout the numerous negative crystals was observed. The simultaneous reflections indicated the crystallographic alignment of the inclusions with the ruby host, evidence that supports their identification as negative crystals.

This simple technique illustrates how light control can be used to enhance the arsenal of tools available for gemological investigation.

Nathan Renfro