This advice is completely legitimate.

Diamond pricing jumps sharply at magic weight benchmarks like 1.0 carat, so buying a 0.9 carat stone can easily save you 20% to 40% while looking virtually identical to the naked eye. He is also spot on about lab-grown diamonds. They are chemically and optically identical to mined diamonds, made of pure carbon with a 10 on the Mohs scale, but usually cost 60% to 85% less.