Never judge a gem by its cover – a guide to identifying stones
(A beautiful and rare Brazilian Paraíba tourmaline, often confused with other gemstones)
Prior to the advent of modern gemmological testing, many gemstones were mistakenly identified as something they were not. Eduard Gübelin (1913-2005), the ‘father’ of modern gemmology, was the man widely accredited with devising the laboratory techniques to distinguish one stone from another. He achieved this largely by painstakingly cataloguing many hundreds of thousands of gems from around the globe into a library collection that is still used as the basis for much gemmological research to this day. A commonplace example of the confusion of two similar stones is that of ruby and red spinel, a situation that is made even more complex by virtue of the fact the two gems are often produced in the same regions, notably Burma (Myanmar) in South East Asia. Relying on colour alone to determine the identity of a gemstone has often lead to cases of mistaken identity, as illustrated by numerous historical incidences. Perhaps the best known example is that of the infamous Black Prince’s Ruby. Originating in Spain in the 14th century, where it was seized by the forces of King Pedro of Castille from the Moorish Prince Mohammed of Granada as a spoil from a battle victory. This large, semi-polished gemstone had the approximate dimensions of an egg. It found its way from one royal court to another throughout the Medieval period, finally resting with the English monarchy, where it remains set in the Imperial State Crown to this day.
(The Black Prince Ruby, centrepiece of the British Imperial State Crown, is actually red spinel)
However, it was not until the early twentieth century that it was discovered to be spinel! Intriguingly Agatha Christie’s famous Belgian sleuth, Hercule Poirot explained the method of discerning spinel from ruby: “observe the gem under the polarizing filter while I shine a light through the stone. At the same time, I will rotate the filter and turn the stone in several directions to avoid looking down the optic axis. Does the gem blink or change colour? No, it does not. Let me explain. If this were a ruby it would blink orangey-red to purplish-red. So you see, this red stone is an imposter! It could be a spinel or red garnet.” Herein lies the key differences between ruby, spinel and garnet, which, to the cursory glance of the untrained eye, appear almost identical in every respect. Crucially, when light penetrates a ruby each ray is split in a process known as double refraction, presenting the ruby as two different colours (in a phenomena known as pleochroism), those of orangey-red and violet-red. However, there is one aspect (or angle) of a ruby in which it will appear as one colour, whereas to view the pleochroism the stone must be orientated in at least three different positions. Red spinel and garnet, by contrast, will present the same colour in all orientations of the stone and as such they are said to singly refractive gemstones. Thus single refraction is the key indicator that is stone is red spinel or garnet, rather than ruby.
(Many subtle differences require laboratory equipment to discern)
Double refraction is not unique to ruby, it is also a critical identification feature of sapphire (both of the corundum family). Citrine, tourmaline and topaz can also show a very shallow double refraction but this can only be detected under laboratory conditions. Emeralds can appear very similar to tsavorite garnet but these two stones can also be separated from one another by the use of a small light source and a loupe, to observe the presence of the doubly refractive pleochroism in the emerald’s light characteristics. Much can also be determined from a stone’s lustre, even with the naked eye. For example, an emerald’s lustre is said to be vitreous, or glassy, to waxy, whereas tsavorite’s is described as greasy to vitreous. Years of experience enables cutters, polishers, master jewellers, gemmologists and gem dealers to discern one stone from another by looking at its optical characteristics and their refractive qualities, in addition to lustre, hue, tone, colour, cut, heft (the comparison of the size and mass of a stone), its facets, fractures and inclusions. The latter can be observed under a loupe in order to determine an emerald from a garnet by virtue that the garnet will likely contain fewer visible inclusions. Lustre can be a useful identification tool when comparing stones such as aventurine quartz and jade, which though similar in colour can be distinguished by the dull appearance of the jade in comparison with the quartz. Transparent hard stones that are similar in appearance, such as yellow sapphire, topaz, garnet and citrine quartz can also be distinguished by their lustre qualities.
(The ‘big’ 3 – ruby, sapphire & emerald – often mistaken for other gems similar in colour)
With experience, a gemmologist can see the difference between the refraction of light through a hard yellow sapphire, which is bright and shiny, whereas soft yellow stones, such as yellow opal and amber, are waxy and dull in their reflective qualities. Sapphires (and other corundum, including rubies) are the hardest coloured gemstones of all, at 9 on the (1-10) Mohs scale of hardness, on which diamond is the only ‘harder’ gem at 10. Topaz and citrine sit at 8, emerald at 7.5, opal at 5-6.5 and amber, the softest coloured gem, at just 2. As you descend the Mohs scale, from 10-1, you observe a corresponding reduction in heft, hence the heft of diamond is considerably greater than that of amber. Both yellow sapphire and topaz are said to produce a vitreous lustre and both display double refraction so differentiating the two can only be done in a laboratory with special equipment. When it comes to distinguishing blue sapphire from other blue gemstones, such as kyanite, benitoite and blue spinel, the presence of double refraction is again key. As with all fields of science, there is no substitute for an experienced eye and many of the skills of the gemmologist take many years to forge. It is only when specialised laboratory equipment, lengthy courses of study and decades of experience combine that it becomes possible to truly master the art of judging a gem among its peers.
(Examples of the position of some gemstones on the Mohs scale)
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