Jewelry returned an average of 1.9 percent in 2014, according to the Coutts Index of "passion" investments published last month. This trumped the return on watches, fine wines and trophy properties, although it was dwarfed by the 40 percent averaged on a "classic" car investment.
This year, jewelry investors may be less lucky, with precious metals prices falling across the board as part of a broad-based decline in commodities. Spot gold prices have fallen 11 percent over the last 12 months, while the price of silver is down a sharp 24 percent and platinum is down 29 percent.
Jewelers are feeling the pinch, with Tiffany & Co.'s second-quarter earnings and sales coming in below expectations. The U.S. luxury retailer flagged that the strength in the U.S. dollar had knocked 7 percent off worldwide sales.
Meanwhile, De Beers, the world-leading diamond producer, posted revenue of $3.0 billion for the first six months of 2015, down 21 percent from the $3.8 billion posted in the same period a year before.
As a result of the weakness, the Republic of Botswana, which owns 15 percent of De Beers, cut its 2015 economic growth forecast to 2.6 percent in August from the 4.9 percent predicted in February.
—By CNBC's Katy Barnato. Follower her on Twitter @KatyBarnato.