Man-made diamonds have come to the point where they cannot be distinguished from natural diamonds:

Diamonds on Demand

From the article:

And yet diamonds are simply crystallized pure carbon, just as rock candy is crystallized sugar—an ordered array of atoms or molecules. Another form of pure carbon is graphite, but its atoms are held together in sheets rather than rigidly attached in a crystal, so the carbon sloughs off easily, say, at the tip of a pencil. Thanks to the strength of the bonds between its carbon atoms, diamond has exceptional physical properties. It’s the hardest known material, of course, and it doesn’t react chemically with other substances. Moreover, it’s fully transparent to many wavelengths of light, is an excellent electrical insulator and semiconductor, and can be tweaked to hold an electrical charge.

It’s because of these admittedly unglamorous properties that lab-produced diamonds have the potential to dramatically change technology, perhaps becoming as significant as steel or silicon in electronics and computing. The stones are already being used in loudspeakers (their stiffness makes for an excellent tweeter), cosmetic skin exfoliants (tiny diamond grains act as very sharp scalpels) and in high-end cutting tools for granite and marble (a diamond can cut any other substance). With a cheap, ready supply of diamonds, engineers hope to make everything from higher-powered lasers to more durable power grids. They foresee razor-thin computers, wristwatch-size cellphones and digital recording devices that would let you hold thousands of movies in the palm of your hand. “People associate the word diamond with something singular, a stone or a gem,” says Jim Davidson, an electrical engineering professor at Vanderbilt University in Tennessee. “But the real utility is going to be the fact that you can deposit diamond as a layer, making possible mass production and having implications for every technology in electronics.”

An overview of the Gemesis technique:

The Apollo technique: “We Grow Diamonds” (CBS News)

For more info see: How diamonds work