The emerald cut is a type of step cut, probably developed from the table cut or trap cut. Most likely, it was an attempt to improve the faceting of the nine or ten-facet table cut and took place around 500 years ago.
As the name suggests, the emerald cut was originally popular for cutting emeralds because of the nature of the shape of beryl crystals, which form in the hexagonal system as square or longish rectangular shapes. Cutting the emerald into a square or rectangle therefore allowed for maximum yield from the rough.
The emerald cut also comes as a square, called the square emerald cut. The Asscher cut, patented during the Art Deco period, is one version of this - albeit with larger corners.
Indeed, it was during this period that many famous emerald cut diamonds were faceted including the 142.90-carat Jonker, cut in 1935 with 66 facets. Reputedly to make the stone more oblong, it was later recut to 58 facets and 125.65-carats.
Famous jewellery incorporating the emerald cut includes the Gordon Necklace, its pendant set with a 50-carat emerald-cut sapphire within a clustered frame of 10 marquise-cut diamonds, 26 pear-cut diamonds, 7 round brilliant cut diamonds and 25 baguette-shaped diamonds.
Characteristically, the emerald cut has an octagonal table facet and girdle surrounded by a series of parallel four-sided facets, and each of these are joined by usually the same number of corner facets - although, to save as much weight as possible, the additional corner facets do not always continue all the way.
The number of facets is determined by the faceter, but generally there can be more facets for larger-sized gems. The usual amount of facets is 49, plus or minus a culet. In larger stones, sets of four facets are sometimes cut, while smaller stones or those with large tables commonly show sets of two crown facets.
Emerald-cut stones do not have a culet. Instead they have a keel, which runs the length of the lowest part of the pavilion. Ideally, the keel length is the same as the stone length, minus the stone width. This makes it possible for the stone to have a concentric pattern sometimes referred to as a drop cut, which gives one a feeling of vertigo. The light return of these stones is thus less important than the effectiveness of the patterns and scintillation.
Reputed for its understated, sophisticated look, the emerald cut has less flash - reflection and refraction - than brilliant cuts; however, the larger facets are renowned for showing large, strong flashes of fire.
Because it is flat-sided, this cut lends itself to easy pairing with a range of side stone shapes. But this cut does have its limitations. The quality of the diamond must be fairly high, as less faceting means inclusions are much easier to see - VS2 stones are often not eye clean. Colour also shows up rather too well so it is advisable to look sideways, lengthways and diagonally when colour grading an emerald-cut diamond from the face down position.
A good length-to-width ratio is also important when selecting quality emerald cuts. This will help avoid the "bow-tie effect" found in all non-round diamonds - two triangular shadows in the centre of the stone where light leaks out from the bottom.
According to popular opinion, the ideal emerald cut should have its length exceeding its width by 30 to 60 per cent. While leading labs say a length between 1.5 and 1.75 times the width, a length of 1.2 times the width is also quite popular on stones with large corners.