Diamonds can have visible inclusions and flaws as is the case for most gems and it is the decision of the diamond cutter regarding which inclusions can be kept and which of those must go. The structure and arrangement of the crystal also decide whether or not the diamond will be able to hold itself together. Despite it being a stone known for its hardness, it can shatter from the impact of a single blow which makes it quite a task; figuring out the right spots to create facets on the stone.
(Clockwise: The Koh-i-noor diamond, The crown of the British monarch, Queen Victoria and Queen Elizabeth II, images courtesy of Pinterest and Getty Images.)
The cut of the diamond can also result in a significant reduction of the weight of the stone. An example of a diamond losing a significant amount of weight after being cut is the Koh-I-Noor. The Koh-I-Noor weighed 186 carats originally and after being re-cut, it weighed 108.93 carats which means it lost about 43% of its original weight.
(Diamond cutting; image courtesy of GIA edu.)
The setting of the diamond is also taken into consideration, meaning, if it will be set in a piece of jewellery, whether or not it will be accompanied by other stones, the shape and size and colour of the other gems. Also taken into consideration is the faceting of the stone, how the light will reflect off of it. Shapes for the diamond are considered. Some of which are, oval, round, teardrop or pear-shaped, heart-shaped, cushion cut, to name a few.
Judging the hardness of a stone is a good way of figuring out if a stone is, in fact, a diamond, however, the downside is that a diamond can scratch another diamond so there is a possibility of inflicting damage on a perfectly good stone. In place of that scratch test, is a test on thermal conductivity that can be performed to judge the authenticity of a diamond. A pair of thermistors (a resistor that relies on temperature) is mounted on a copper tipped wire and one thermistor is used to create heat and the other to measure the temperature, if the stone conducts the energy in a matter of seconds, it proves its authenticity as a diamond.
After the faceting is done with, the diamond is polished; this is a rather long and tedious process and is taken on by technicians proficient in the procedure. The diamond is re-examined for flaws and inclusions which can then be removed if deemed necessary.