Hatton Garden diamonds: stunning new Diamond Halo Earrings set in 18ct Yellow Gold: fabulous elegance and impact: GIA certificated

How to choose your diamonds? Here we present a brief guide to the qualities of gemstones, and how to buy them, starting with diamonds.

DIAMONDS

The value of a diamond depends on an assessment of the four Cs—that is, carat, cut, clarity and colour. Carat indicates the weight of the stone; a carat corresponds to 200 milligrams (about 1/140th of an ounce).

Cut refers to the cutting of the diamond. The cut should be of excellent quality with facets at specific angles so that it perfectly reflects the light and creates a ‘fire’ inside. A diamond with at least 56 facets, a table and a point may be called a brilliant. That does not mean other cuts are less precious, for there are many ways to cut a stone and to bring the best out of it. Whether round, tear-shaped, oval or cone-shaped, the most important requirement is that the cut should be appropriate to the stone.

Clarity refers to the purity of the stone. The number, size, and location of any inclusions, transparency, and possible structural irregularities are assessed. In addition, a distinction is made between defects visible to the naked eye and those that can be seen by a 10-power magnifying loupe. The highest grading is FL for ‘flawless’. Then comes IF, ‘internally flawless’, meaning that no inclusions are visible with the same magnifier. The grades below include VVS, ‘very very slightly imperfect’, and VS, ‘very slightly imperfect’ with inclusions invisible to the eye, visible only with some difficulty with the magnifier; these do not affect the stone’s brilliance. SI, ‘slightly imperfect’ and I, ‘imperfect’, have inclusions that can be seen with the naked eye, and may affect the stone’s brilliance.

Diamonds are graded for colour, with the ideal being colourless. This is an important, sought-after characteristic because it allows the white light to penetrate the stone unhindered, emerging as a rainbow. Most stones are situated between ‘D’, which is ‘colourless’, and ‘Z’, which is light yellow. The colour is determined by comparing the stone with an internationally fixed number of master stones. So-called ‘fancy’ diamonds which may be blue, red, green or even black, are another story.

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