How to learn to tell an expensive ruby from a cheap imitation

Red spinels, tourmalines, rubies and synthetic rubies may look identical to the naked eye. Yet their prices can vary from US$300 to US$3,000 per carat. So how can you tell them apart?

I recently joined a four-hour class “Recognise the Gemstones” run by L’École School of Jewelry Arts, a subsidiary of Van Cleef & Arpels, to learn how to tell a ruby from the others.

Rubies are rare and expensive gemstones, with high levels of hardness and durability. – Spinels, tourmalines and synthetic rubies – can all possess the same blood-red hue and are used as substitutes for natural rubies.

After a short introduction, we are tasked to spot subtle differences between four similar gems, using tweezers, 10x magnification lenses, and some specialised equipment: a dichroscope, a refractometer, a polariscope, a spectroscope and a density scale. But first, we must use a tool that is familiar to us all: our eyes.

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We need to look for what gemologists call “inclusion”, a term to describe fragments of another material inside a host rock (or in this case, gemstone). Some have inclusions shaped like clouds, others needles.

Lab-growngems appear the same as their natural counterparts in every measurement, since they have identical chemical compositions. But artificial rubies are revealed by their clarity.