DeBeers’ slogan is “A diamond is forever.”
Ian Fleming’s 1956 novel was entitled “Diamonds are forever.”
And as Cecil noted at http://www.straightdope.com/mailbag/mdiamond.html , in 1925, Anita Loos wrote that “a diamond and safire (sic) bracelet lasts forever.”
Who linked diamonds and forever first? Clearly, Fleming’s out if the running, losing out to Loos by 31 years. But does anyone know when DeBeers starting using their slogan?
According to this site
DeBeers started using the slogan after WWII. They wanted Americans to spend their expendible income on diamonds, so they hired an agency (N.Y. Ayer) to create a bold diamond campaign. So, N.Y. Ayer was ultimately responsible for DeBeers’ enduring slogan.
As for who originally liked diamonds to forever, I couldn’t even begin to guess. I always thought it originated with DeBeers…
And once again, I point out that diamonds are not forever. They’re a lousy long-term investment.
Diamonds begin to decay the moment they are released from the enormous heat and pressure that formed them. They may not be forever, but they’ll last a good few hundred thousand years 
Asterion, are you pointing out that diamonds depreciate rapidly?
Unless you put them in a high-temperature oven (at normal atmospheric pressure). Then they’ll last a good few minutes. 
This also increases their rate of depreciation.
Diamonds aren’t really forever. But paying for them is.