Picture courtesy of http://www.pm-review.com/avure-technologies-launches-its-new-terapi-hot-isostatic-press-a-tera-sized-hip-at-world-pm2012-yokohama/
I worked at GE Imaging Systems and we had a machine called a Hot Isostatic Press. This badboy could turn a powder into garnett. It worked by introducing amazing vacuum pressures and super high heat in a controlled and mapped, repeatable process. never tried but it had the capacity to make diamond from a carbon source, like graphene. It is an AMAZINGLY EXPENSIVE INSTRUMENT, but they exist and can definitely produce diamonds, although it may take a composite of sulfur or another non carbon atom to make a diamond of significant size. Its color would be altered, if that route of synthesis is used.