In an age when the global wealthy are looking for portable, secure stores of wealth, diamonds should shine.
After all, diamonds are a scarce commodity. Like vintage cars and fine art, they are stores of value that can also be enjoyed. And as the recent jewel heists in Cannes show, diamonds are easy—perhaps too easy—to transport.
But valuing diamonds and the broader "market" for the gems is a virtually impossible task. That makes diamond investing a risky sport best left to insiders and seasoned professionals.
(Read more: The world's richest diamond owners)
ConvergEx Group points out that, like many other commodities, diamonds have hit a rough patch.
"As it turns out, 2013 hasn't been all that good for sparkly bits of carbon either, with prices flat to down 1 to 2 percent. Diamond prices actually peaked in early to mid-2012 and are down 8.5 to 9.6 percent since then," writes Nicholas Colas, chief market strategist at ConvergEx Group, a global brokerage based in New York.
But the numbers aren't that simple. While the diamond-industry source Rapaport tracks prices for various categories of diamonds, the idea of an authoritative index of diamond prices remains elusive. That's because prices vary widely by size, color and quality.
Prices can also swing wildly day to day given the vagaries of the famously opaque market, according to Mahyar Makhzani, the London- and Geneva-based managing director of the Sciens Colored Diamond Fund.
"Everyone is always asking me if there is a reliable index for diamonds, and the answer is it's impossible," he said. "We just don't have enough transparency."
Since 2011, Makhzani said, the diamond market has had three big swings, in which prices have risen at least 5 percent or fallen at least 10 percent.
From a financial perspective, he said, there are two broad types of diamonds: consumption goods and investments.
Colorless diamonds are generally best for gifts, he said, while colored diamonds (pink, yellow, etc.) can be "assets with highly attractive investment" potential. Prices for a number of smaller colorless diamonds have fallen, while those for many colored diamonds have been stronger.
Makhzani's fund, which focuses on ultra-rare colored investment diamonds, is up 26 percent this year. It ended last year up 24 percent.
Giant rare diamonds are also doing well. The 101.73-carat diamond that went for $27 million at a Christie's auction in May would probably sell today for "maybe $30 million or more," Makhzani said.
(Read more: Huge diamond sold at auction for $26.7 million)
"There just aren't that many 101-carat stones out there," he said.
Nor are there that many people with $27 million to spare for a single jewel.
—By CNBC's Robert Frank. Follow him on Twitter @robtfrank.