Their marketing works just as well to create the diamond market just as well as it does to make synthetic diamonds mean you don't really love her.
There may be market for artisanaly doped synthetics by customer design or something.
You ever watch a sitcom where a future mother-in-law comments on how big the diamond in the ring is? Next time she'll be disappointing it's synthetic, except they'll come up with a more negative term (artificial, fake, etc.).