Both individual and institutional investors have been looking harder at diamonds recently. Individual stones — white and color — are gaining popularity because they are tangible and portable, and they promise steady appreciation, even if that promise is not always realistic. Diamond funds and mining stocks also have their advocates.

According to a 2013 report by Bain & Co., increased demand from populous, developing countries will lead to a compounded annual rise in diamond prices of about 5.1 percent between 2017 and 2023. Color diamond prices could rise even more dramatically.

Expert opinions on the credibility of diamonds as an investment vary greatly, however. One problem in assessing the category is valuation of the individual stone, which is highly subjective. Another difficulty is that the category must bow to the whims of fashion, especially with regard to color diamonds. A third is that supply, which largely drives price, is unpredictable over the long