The word "carat", when referring to the weight of a diamond, comes from the Greek word keration, or carob bean, a unit that has been used to measure precious stones since the 16th century.One carat equals one bean, roughly, and while many an aspiring bride today knows exactly what a one-carat diamond might look like on her finger, few of us are familiar with the dimensions of a single carob bean, let alone 163.41 of them. (I tracked one down, and it?s about the size of a petit pois.)

A whopping 163.41ct ? that?s the official size of the gigantic emerald-cut stone anchoring a new necklace by Swiss jewellery house de Grisogono as the brand celebrates its 25th anniversary. Fittingly flamboyant, the necklace itself is a huge, swooping collar of emeralds and diamonds, with the main event on a detachable setting at its base. And while the diamond is by far the largest the brand has had the privilege of working with, the necklace is typically "de Gris" (as it?s affectionately known by its jet-set clients), bold in colour and scale and high on glamour. The whole thing is named "the Art of de Grisogono ? Beyond Jewellery, Creation 1", and while that?s quite the name for a necklace, it?s also quite a piece, and it is set to be auctioned at the Magnificent Jewels sale at Christie?s, Geneva, on 13 November. The finished product will have taken nearly two years to get to that point: the 404ct rough diamond it started as was discovered in an Angolan mine in...