This is true with some qualifications. If you're interested in the kind of investment grade diamonds that a major auction house would deal with, then you're looking at heavy weights and/or fancy colors that synthetics can't reach yet. In the diamond trade the word "paragon" is sometimes reserved for flawless or near-flawless stones above 100 carats, of which there is a long list of famous examples, but the largest gem grade synthetic is still around 30 carats I believe. Vivid colors top out at much lighter than that. I guess we'll be able to outdo nature within a few decades though (as far as terrestrial diamonds go, anyway -- I seem to recall reading somewhere about the discovery of moon-sized space diamonds).