An emerald is a gemstone, containing traces of chromium which give them their characteristic green colour. Sometimes vanadium may also be present in the stones. The toughness of emeralds is generally quite low (in comparison with other gem stones) due to the typically high number of inclusions, which lends them to being damaged more easily.
The quality of an emerald is graded in the same way as other gemstones. Carat weight, colour, clarity and cut determine the value of a stone, with colour being the most important of the four. The best emeralds are highly transparent with a pure verdant green hue.
Emeralds have a primary hue of green with secondary hues ranging from yellow to blue. The mineral beryl, from which emeralds occur when chromium is present has a hardness of 7.5 to 8 on the Moh’s scale. Gems with light tones are called ‘green beryl’, emerald is the name used when the tone is medium to dark.
Where 0 would indicate colourless and 100 would be completely opaque on a scale describing tone, a value of around 75 is considered the optimum for the finest emeralds. In this range, the better stones would be well saturated in colour and bright.
Beryl is the basis for a number of other gems where the colour is
dictated by the presence of minerals other than chromium. Bixbite is red
beryl, Heliodor is yellow beryl, Aquamarine is blue beryl and Moganite
is pink beryl.
Whereas diamonds are graded under a 10x magnification, emeralds are graded by the naked eye. They tend to have fissures that break the surface and a high number of inclusions, an emerald without fissures breaking the surface is incredibly rare. These stones are treated (oiled) to improve the clarity of the gem. An emerald without inclusions visible to the naked eye is considered flawless.
The mossy look of an emerald is due to the surface fissures and inclusions and is commonly referred to as ‘the garden’. This can be used to uniquely identify a stone, as no two are exactly alike. Generally emeralds are cut into cabochons as opposed to faceted shapes like a diamond. A rectangular cut with the top edges faceted is the signature ‘emerald cut’.
Cedar oil is commonly used to (enhance) fill in the cracks, which reach the surface of an emerald after it has been cut and polished. This helps to build their strength and improves the clarity of the stone. The refractive index of the cedar oil is similar to that of an emerald making it the preferred choice, although synthetic oils are also used. The emeralds are rated according to the level of enhancement on a 4 step scale: none, minor, moderate and high. Two stones with similar ‘gardens’ can be priced very differently, if the level of the enhancements of one gemstone is none (expensive) and the other is moderate or high.
Synthetic emeralds have been made by either hydrothermal or flux-growth techniques since the 1960’s. Flux-growth emeralds are grown on colourless beryl seeds, where they have been coated on both sides. The growth occurs at around 1mm per month, so it takes about 5 months to produce a crystal of 5mm thickness. The hydrothermal method involves acidic conditions where a diffusion-reaction process is used in conjunction with convection on growth crystals. No, I don’t know what that means either!
Heliodor (Golden Beryl) ranges in colours from a pale yellow through to a brilliant gold, caused by the presence of Fe3+ ions in the crystal. This crystal tends to have very few flaws when compared to emerald and a flawless 2054 carat cut stone is on display in the Hall of Gems in Washington DC.
Red Beryl is quite rare and has only been found in a few locations. The red colour, typically dark red is connected with the Mn3+ ions in the crystal. This gemstone occurs in topaz-bearing volcanic rocks, the largest deposits being found in the Wah Wah Mountains of mid-western Utah.
Morganite (Pink or Rose Beryl) has been found in Madagascar and
California. It is quite rare and it varies between light pink and rose
in colour. Orange and yellow varieties have also been found and bands of
colour are not uncommon. Heat treatment is used to reduce the patches of
yellow and the pink hue is caused by the presence of Mn2+ ions.
Aquamarine is beryl with a primary hue of turquoise or blue. It occurs widely in areas where ordinary beryl is found and the light blue colour is created by Fe2+, dark blue when both Fe2+ and Fe3+ ions are present. Maxine is a deep blue version of aquamarine, but its colour completely fades when heat treated or exposed to sunlight.