Diamonds, as the song says, are a girl’s best friend. But certain diamond jewelry — like rings, bracelets and earrings — can be a great investment.

Of course, not all brands make great pieces all of the time (and many lesser-known designers have produced fantastic pieces, like Mr. Webb’s opal necklace, that stand out). But for the big houses, time periods matter for investing.

“Of all the periods people want to collect, I always tell them Art Deco of the 1920s and 1930s,” said Diane Lloyde Roth, owner of L’Armoire, a store in New Canaan, Conn., that has been selling investment-quality jewelry for three decades. It “was probably the most beautiful period of all in jewelry production.”