A well known London-based company dealing in Pakistan's Swat Valley emeralds was asked by one of its clients to provide an engagement ring stone. The company concerned specialises in melee stones and was unable to provide a stone of the size required. To cut a long story short, through an intermediary, I was asked if I could cut a stone for them. The client was adamant that she wanted a round stone despite the fact that I pointed out that round designs including the ubiquitous Standard Round Brilliant provide very poor yield from typical emerald rough and many, including the SRB do not optimise the magnificent colour of Swat emerald. Nevertheless, a round it had to be and both the client and the company were intrigued by the idea of a bespoke design that would provide similar light performance to the SRB but with a better yield and better colour. A large number of designs were created and optimised for emerald, four of which are shown above. Of all, the one shown bottom left provides the best yield and colour and luckily this is the design preferred by both the client and the company. Overall it has light performance equivalent to and in some respects superior to the SRB. SRB graphs left, MLB graphs right.
Face up windowing is slightly better in the SRB but there is substantially greater head shadow. Also, to achieve the SRB optical performance a low crown was required. This would result in diminished colour in the finished stone and a stone of considerably lighter weight than the MLB with its higher crown and deeper pavilion. In fact, other than colour considerations the MLB produces a stone that is approximately 20% heavier than the equivalent SRB. The MLB differs from the SRB in having larger and radial pavilion mains. Crown facets are mirror image symmetry but have a curved relationship between them rather that the squarer arrangement seen in the SRB.