Information from:
The Jeweller’s Directory of Gemstones

Advertising tells us that a diamond is a rare and valuable thing to possess. While it cannot be disputed that white diamonds remain the most important gemstone within the jewellery, it is worth bearing in mind that diamonds are mined throughout the world on a massive scale. The only restriction to the supply comes in the form of the cartel that controls it. As for the high price, it is the result of a careful marketing strategy, not rarity.

The diamonds industry works quite separately from the coloured gemstone market; it has a culture and structure that doesn’t exist anywhere else int he gemstone business. Not only do diamonds dwarf the trade in other gemstones by being the most mined of gems, they are also the most carefully classified and tightly controlled. Unlike coloured stones, a detailed grading system based on carefully guarded master samples ensures that the factors used to dictate quality and value remain constant.

Diamonds were first cut to improve their look in the 13th century. Prior to that,t he rough edges of diamond crystals were simply smoothed down and the flat surfaces polished. The cut used was the extremely basic table cut, in which an octahedral crystal was given a flat surface on top (called the table) and a smaller flat surface underneath (called the culet). In the late 1400s, the cutting wheel appeared and diamonds started to get their rounded outline.

Widely used from the early 1600s, the rose cut was an economical way to cut flat diamond rough with little wastage. The stone had a faceted dome and a flat base under which gold or silver foil could be placed to reflect light, giving diamonds a soft lazy fire that drew the eye into the stone. This method of cutting and setting diamonds had been used int he Far East for centuries; in Asia, simply cut kundan stones were set, like cabochons, with pure gold foil, and flat, angular diamond crystals were used with coloured gemstones beads and natural pearls.

At the end of the 17th century a Venetian cutter, Vicenzio Peruzzi, created what is known as the old cut. Although more irregular in shape, it is viewed as being closest to the more brilliant cut. The old cut enjoyed considerable popularity; its 58 facets gave it life and fire, while the overall proportions brought a subtlety and depth to the gemstone.

The modern brilliant cut was introduced in 1919 and was viewed by the industry as the ideal cut. It was very popular with jewellery makers and buyers alike because it made the most of the gem’s naturally strong light dispersion, resulting in tremendous brightness and fire. The brilliant cut became the standard for the trade.

Id a diamond is not a brilliant cut, it is a fancy cut. This category includes baguettes, pear shapes, heart shapes and marquises. Fancy cuts are not produced in large quantities and often have a higher carat price because they entail more wastage of rough material. A baguette or emerald-cut diamonds has a lightly lower brilliants than a brilliant-cut diamonds because the step-cut style was primarily intended for coloured gemstones, the large table facet showing off the coloured and the step-cut pavilion facets allowing light into the back of a stone. Modern cutting technology has brought us many new fancy diamonds cuts; the Asscher, Trilliant, princess and baguette cuts have all been adapted from the brilliant cut.

Grading factors for diamonds

Clarity: The clarity grade will identify how clean a diamond is as seen through a 10x loupe. Tiny cracks that look like feathers, microscopic diamond crystals and carbon deposits are common in diamonds and generally won’t affect the brilliants. They will, however, affect the price.

Colour: Colour is best observed by examining the stone through the pavilion, with the table upside down on folded white card-stock. if the stone is viewed through the table,t he brilliance will hide the colour. Grade D carries a significant premium, yet you will be unlikely to see any difference between Grades D and E, and the price difference could pay for a larger diamond with better clarity.

Weight: Diamonds are weighed to one-hundredth of a carat and are priced according to their weight. To compare the prices of two diamonds you refer tot he carat price. There are 100 points to a carat and 75 points equals 0.75 carat (three-quarters of a carat). Weight also relates to size. For instance, a 1-carat brilliant-cut diamonds should measure 6.5 mm in diameter. If the stone is cut slightly shallow, the diameter will be larger; if the cut is too deep, the diameter will be smaller.

Cut and Shape: The cut, also called the make, includes proportions and symmetry and has the greatest influence on the brilliance of a diamond. Poor cutting, such as a thin girdle, will affect the durability of a diamond, making it more susceptible to breaking or chipping. A sloping table, misaligned facets or poor symmetry will prevent light travelling through the stone properly and so affect the brilliance. Cutting faults will also cause significant problems when it comes to setting the diamond.

Fluorescence: A stone that produces a colour reaction when exposed to ultraviolet light is described as having fluorescence. If a diamond fluoresces a blue colour, it will appear whiter in daylight, masking any yellow tint it may have. This is useful if the diamond had a low colour grade, but a white diamond that fluoresces yellow will have a yellow tint in daylight - not what you want in a fine white stone!

Proportions and Finish: In a brilliant cut,t he crown (the top portion of the stone) should measure one-third of the depth of the pavilion (the distance from the girdle to the culet). Good proportions create brilliance by allowing the light to be reflected up through the stone’s table. The proportions of a table width to stone width determines a stone’s fire: a smaller table produces more fire while a bigger table produces more brilliance. Poor proportions can lead to a dark bow-tie or butterfly effect across the centre of the stone in ovals and fancy cuts. This diminishes the beauty of the stone and devalues the diamonds.

Natural coloured diamonds

Yellow, orange, brown and black are the most common natural diamond colours and easy to find. Pink, light green and lavender are rarer, but can still be sourced without too much effort. Deep blue, red and dark green are extremely rare and seldom seen; they are the most expensive of all the natural diamonds colours and fetch high collector prices.