A diamond will lose 40-80 % of its weight during the cutting process!

A diamond’s beauty essentially comes from its cut and the skill of the diamond cutter. The fire and sparkle you see is the result of a beautifully cut diamond.

In order to achieve this fire and sparkle, the diamond should reflect all the light that enters it and reflect it back up through the top of the stone.  Highly skilled diamond cutters, blockers and brillianteers complete this process.

The following are some methods used in order to create a Round Brilliant Cut Diamond

Marking: The first and most important step to diamond cutting is to inspect the rough diamond to determine how to preserve the most amount of weight, while maintaining a high quality stone.

Due to its crystalline structure, diamonds have a grain or cleave.  This will make the rough stone more susceptible to cracking during the cleaving/sawing process if not properly marked.

Cleaving: This is a time-consuming process that must be done with great care, for if struck at the wrong point, the diamond can shatter.  A chisel is inserted, and with a sharp blow the diamond is separated into (ideally) 2 pieces. Some stones are determined to have too many stress points to be cleaved, in which case it will go directly from the marking stage to the sawing stage.

Sawing: With the invention of the Diamond Saw, it was now possible to cut against a diamonds natural grain without shattering it. This gave diamond cutters more freedom when determining how to cut a stone.  The rough stone is place on a circular saw that is laced with diamond dust and lubricated with oil. Depending on the size of the stone, this could take between hours and days.

Faceting/Cutting: The rough diamond is placed on a lathe while another diamond is held against it to create the table and the cone-shaped bottom of the diamond that will soon become the pavilion. 

It is first given to the “blocker’ sometimes called a “lapper”, whose specialty is giving the rough diamond its first 18 facets.

The “brillianteer” will put the final touches on the diamond and will ultimately determine the amount of brilliance a diamond will have by placing and polishing the final 40 facets completing the faceting of the diamond.

Polishing: This process has been going on in one form or another since 14th century India. Current times use a “lap” which is a circular disc that is laced with diamond dust and oil. This not only polishes each of the 58 facets, but also works as an abrasive to buff away surface flaws, unleashing the brilliance and sparkle of the gem!

Sources:
http://www.edwardjayepstein.com/diamond/chap11.htm
http://en.wikipedia.org/wiki/Diamond