So here comes the gemmology lesson!
As a gemmologist I love seeing inclusions, they are natures fingerprints and can help us determine where a gemstone comes from, as some are unique to their geographical origin.
For example, Columbian emeralds have what’s called ‘three phase’ inclusions, that’s a liquid filled cavity, containing a crystal and a gas bubble. Indian emeralds contain ‘two phase’ inclusions – these are comma-shaped liquid containing a gas bubble. Emeralds from Zimbabwe contain tremolite needles. For all buyers, however, only with an accompanying and independent certificate can an emerald with a geographical origin be confirmed.
Like other coloured stones, emeralds are valued on three factors:
- Hue – the colour
- Saturation – the intensity of colour
- Tone – light, medium to dark
The most desirable emerald colours are bluish-green to pure green. As with other coloured stones, it requires a well-trained eye to recognise the subtle variations that make significant differences in emerald value. This is especially true in the higher qualities. If the hue is too yellowish or too bluish, the stone is not emerald, but a different variety of beryl, and its value drops accordingly.
Emerald crystals often have colour zoning and irregular colour distribution. This presents a challenge when cutting the stones.
Saturation and Tone
The more intense the colour the more valuable – the colour should be vivid and saturated, but not too dark. The most-prized emeralds are highly transparent, their colour evenly distributed, with no eye-visible colour zoning.
Emeralds typically contain inclusions that are visible to the naked eye, therefore they are an inherent feature of these gemstones and accepted, even romanticised. Emerald inclusions are often described as looking mossy or garden-like, so they are delightfully referred to as the ‘jardin’, (French for garden). ‘Eye-clean’ emeralds are especially valuable because they’re so rare.
As emeralds are rarely flawless, stones are often oiled to fill and disguise the cracks, hide flaws, and enhance colour. This provides greater beauty and long-term stability of the gemstone and is acceptable if it fully disclosed. The Gemmological Institute of America (GIA) applies a classification system to evaluate the level of treatment, but not to offer an overall clarity grade for the stone. The report describes the level of clarity enhancement as minor (slight oiling), moderate, or significant (fracture filling).
It is for this reason that ultrasonic cleaners must be avoided.
Emeralds are notoriously difficult to cut – the cutter must consider the crystal, rough depth of colour, durability and inclusions when making cutting decisions:
- As the colour is so crucial to the emerald’s values, so the cut must maximise the impact of the hue, tone, and saturation. An expert cutter can affect colour by adjusting an emerald’s proportions and number of facets. For example, a pale stone can appear darker with a deep cut, a small table and fewer facets. A dark stone can be lightened with a shallow cut, a large table and additional facets.
- Emeralds are dichroic (meaning they have two different colours when seen from different directions) blueish-green to yellowish-green. As the blue-green is more prized, so the cutter must align this side with the top (or table facet) of the cut gemstone with the crystal length where this is best seen.
- Weight loss greatly reduces the potential value of the gem, so mistakes must be avoided and hence why weight conservation is so key.
- The clarity also causes challenges – the cutter must design the cut to minimise the effect of the fractures and inclusions.
- Partly due to those fractures, emeralds are more brittle than, for example, sapphires and rubies. They have a hardness of 7.5 on Mohs scale but they are also brittle due to stress and the violent conditions under which they are formed. This makes them vulnerable to damage during cutting, polishing and setting, or even during careless daily wear.
To overcome these challenges this gemstone even has its own signature cut. The ‘Emerald Cut’ maximises the yield from the crystal rough, the depth and step faceting show off the beautiful body colour of the gem and the cut corners help prevent breakage as the claws are positioned over the vulnerable corners when setting the stone. This cut can also be used on other gemstones; it gives that wonderful Art Deco geometric appearance.
Emeralds are often set in a yellow gold claw or bezel, an aesthetic consideration that enriches the yellow tones of the gemstone. Platinum is still a precious metal of choice for superlative setting; however, an expert and experienced jeweller is an absolute must. It is for that reason I source all my emeralds from my friends at Emdico Jewellery