Emerald is the birthstone for May, the gem for the zodiac sign Taurus and the 55th wedding anniversary gemstone .
Emerald is a green variety of beryl, a mineral species that includes aquamarine,morganite and yellow beryl. It’s Mohs Hardness is 7.5 to 8. It’s density is lower than that of diamond, a one-carat emerald will appear larger in size than a one-carat diamond.
Colour is the most important factor in establishing the emerald’s value. 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. The most desirable emerald colors are bluish green to pure green, with vivid color saturation and tone that’s not too dark and is evenly distributed, with no eye-visible color zoning.
Clarity is important, but inclusions are tolerated more in emerald than virtually any other gemstone. Unlike other beryl gems, emeralds often contain inclusions and other flaws. While eye-visible inclusions in higher-quality emeralds are generally accepted, when the inclusions have a negative effect on transparency and clarity, they also dramatically reduce value. Highly transparent emeralds that have no eye-visible inclusions are very rare and are highly valued.
Due to the importance of colour the cut must maximize the effect of hue, tone, and saturation. The cutter can affect color by adjusting an emerald’s proportions and number of facets. The cutter can darken a pale stone with a deep cut, a small table, and fewer facets, or lighten a dark stone with a shallow cut, a large table, and additional facets.
The level of fractures in emeralds make them are more brittle than a gem like corundum. This means they are vulnerable to damage during cutting, polishing, and setting, or even during daily wear. A cutter must design the cut to minimize the effect of those fractures on the finished stone.
The most common cut, the emerald cut is a rectangular or square shape step cut with cut off corners. Emeralds are also cut into a variety of other traditional shapes such as pear, oval and round. Highly transparent emeralds are sometimes given brilliant cuts. Lower grade emeralds are often cut as cabochons or beads.
The term ‘oiling’ refers to the practice of immersing emeralds in a colorless oil or resin (most often cedar oil) in order to enhance color, clarity and stability. This is also often done using a vacuum chamber to assist penetration. Non-standard treatments go beyond this by using colored oils and epoxy-like resins.
These treatments dramatically improve the appearance of the gems, but necessitate special care in cleaning and setting. Steam cleaners, solvents and ultrasonic cleaners can remove the oils, making inclusions that were barely visible stand out in sharp relief. Since emeralds can be restored through re-oiling, the damage is considered to be only temporary.