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.

Next, we learn to use a hand held instrument called dichroscope to see whether a piece of gemstone has more than one colour. Rubies are dichroic, meaning when light enters them it will be split into two distinctive colours. So if a stone shows only one colour, it is not a ruby.

Another way is to use a refractometer, a microscope-like device that measures stones’ refractive index. The index range for a ruby is 1.762-1.778.

The polariscope is used to decide whether a gemstone is singly or doubly refractive. When it is lit from below, the inclusion can be more clearly seen, but when the gemstone is turned in different directions, it changes from dark to light or vice versa. If the tone doesn’t change when being rotated, it is not likely to be a ruby.

We were also shown a spectroscope, a device that shows the colour spectrum of a stone. This is a complicated machine and needs experienced operators.

An instructor also uses a density scale to weigh the stones both in air and water; by applying a simple formula, we can work out the densities, which are unique to particular stones, except synthetic creations.

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By the end of the class, we all label the gemstones correctly. Shame we can’t take any of the samples home.

There are classes at L’École where participants can take their work away, such as the hands-on “From French Jewels to Japanese Lacquer”. Not surprisingly, these are more in demand than the lecture-style sessions. Students work with metal and artificial ivory to create motifs on jewels shaped like butterflies.

L’École was founded in Paris six years ago and Hong Kong hosted the first overseas edition in 2014.

According to Marie Vallanet, president of L’École Van Cleef & Arpels, an increasing number of Hongkongers are attending those classes. This year has seen about 750 students enrolled, up from 500 in 2014.

Those interested in gemstones and their identification can join classes on the history of jewellery, gemstone recognition and accessory crafting. The lessons are two to four hours, cost HK$1,000 to HK2,000 and run until October 1.

To sign up, visit: hk.lecolevancleefarpels.com/en/sessions/the-universe-of-gemstones/recognize-the-gemstones/