Catherine, Princess of Wales, born Catherine Elizabeth Middleton in January 1982, has, since her marriage to Prince William in April 2011, become one of the most closely studied wearers of royal jewellery in the world. From a trade perspective her significance is twofold. First, she has acted as a public custodian of important historic stones from the British royal collection, including the late Princess of Wales's sapphire-and-diamond engagement ring and a number of Cartier and Garrard pieces inherited or loaned through the Queen's collection. Second, she has demonstrably shifted commercial demand: stones, settings, and even relatively obscure designers she has worn in public have repeatedly experienced same-week sales spikes, a phenomenon the trade press calls the 'Kate effect'.

The engagement ring

The most photographed piece associated with the Princess of Wales is the engagement ring originally chosen by Prince Charles for Lady Diana Spencer in 1981 and given to Catherine by Prince William in 2010. The ring is set with a Ceylon (Sri Lankan) blue sapphire of approximately twelve carats, surrounded by fourteen round brilliant-cut diamonds totalling about one carat, in eighteen-carat white gold. It was supplied originally by Garrard, the long-standing crown jeweller, from a stock selection rather than as a bespoke commission, a fact widely reported at the time of the 1981 engagement and noted again in 2010. The stone is a classic, mid-saturation Ceylon blue, of the kind produced in volume from the Ratnapura district and cut to a faceted oval. Its visibility on the Princess's hand has, more than any other single object in the past four decades, fixed the public image of what 'sapphire blue' looks like.

Tiaras and the loaned pieces

For state occasions the Princess of Wales has worn several tiaras from the royal collection, most often the Cartier Halo Tiara of 1936 (worn at her wedding), the Lover's Knot Tiara made by Garrard in 1914 after a design originally produced for Queen Mary, and the Lotus Flower Tiara, also Garrard, made from a Queen Mary necklace in 1923. The Lover's Knot is of particular trade interest: pearls of significant size suspended in nineteen interlocking diamond-set lover's-knot bows, demanding considerable skill in settings and a balance carriage of the head, and the piece has attracted scholarly study in the literature of the Royal Collection Trust.

The 'Kate effect'

The commercial influence of the Princess of Wales on jewellery sales has been documented repeatedly in industry reporting and in the broader fashion trade press. Pieces from Asprey, Mappin & Webb, Kiki McMillan, Cassandra Goad, Soru, Annoushka Ducas, Robinson Pelham (who supplied the bridal earrings) and Catherine Zoraida have sold out within hours of public appearances. The pattern is now familiar: a single appearance, a still photograph in mainstream press coverage, and a wave of orders that often outstrips the small workshops' capacity to fill them. The phenomenon resembles the older 'Diana effect' but operates faster and across a wider range of price points.

Stylistic preferences

The Princess's documented stylistic preferences run toward classical proportions: Ceylon blue sapphire, white diamond, South Sea and Akoya pearl, with relatively restrained use of coloured stones beyond sapphire. She has worn citrine, peridot, aquamarine, tanzanite, garnet and amethyst on appropriate occasions but tends to favour the cooler palette. Settings have been principally white metal, frequently with delicate, almost Edwardian, garlandwork. The choice has subtly shifted bridal-market preferences toward halo-style sapphire-and-diamond rings and away from the all-white solitaire that dominated the 1990s.

Custodianship and the future

From the perspective of the gem and jewellery historian, the Princess of Wales is currently a custodian rather than an owner of most of the historic pieces she wears, with the engagement ring and a smaller body of personal pieces standing as exceptions. The Royal Collection Trust catalogues the loaned items and they will, in due course, return to the central collection or pass through the constitutional system of inheritance. Her wearing of them, however, has placed sustained twenty-first-century attention on the workmanship of Garrard, Cartier, and the other firms that built the collection, and has helped to keep the tradition of sapphire-and-diamond royal jewellery commercially relevant in a market that has, in other categories, drifted away from coloured stones.